Putin strikes a tough tone in his first address since the uprising started.

“Those who organized and prepared the armed rebellion, those who raised weapons against comrades in arms, betrayed Russia,” Mr. Putin said. “And they will answer for this.”

Mr. Putin did not utter Mr. Prigozhin’s name, but he referred to him as a traitor, saying: “Exorbitant ambitions and personal interests have led to treason.”

Mr. Putin added that he had ordered “decisive actions to stabilize the situation in Rostov-on-Don,” the southwestern Russian city of a million where Mr. Prigozhin’s forces appear to hold military control.

“The work of civilian and military governing institutions is practically blocked” in the city, Mr. Putin said. “As president of Russia and the commander in chief, as a citizen of Russia, I will do everything to defend the country.”

It was unclear how much the Wagner forces controlled or how much of a threat they posed to the Kremlin. But the confrontation already amounted to the biggest challenge to Mr. Putin’s authority since Russia invaded Ukraine 16 months ago.

In an address to the nation, Mr. Putin called the Wagner forces’ actions “a stab in the back of our country and our people” and vowed “decisive actions.”

Here’s what we know.

What’s happening?

Tension escalated late Friday after Mr. Prigozhin accused the Russian military of attacking his fighters’ encampments — a claim that could not be immediately verified. He also described the invasion of Ukraine as a “racket” perpetrated by a corrupt Russian elite.

Mr. Prigozhin vowed that what he said was his 25,000-strong mercenary force would go on the offensive against the Russian defense ministry, though he said that the actions were not a “military coup.”

The Russian authorities responded by charging Mr. Prigozhin with “organizing an armed rebellion.” A Russian general urged Mr. Prigozhin’s fighters not to “play into the hands” of an enemy that he said was waiting for Russia’s internal political situation to worsen.

Video footage showed armored vehicles from the Russian military in the southern Russian city of Rostov-on-Don, near the war’s front line in Ukraine where Mr. Prigozhin’s fighters had been operating.

Additional videos circulating online and verified by The New York Times showed dozens of soldiers getting out of military vehicles and pointing their guns at the compound that forms the military command post in southern Russia.

Early Saturday, the governor of the Rostov region asked residents to stay in their homes, saying that the authorities were “doing everything necessary” to ensure their safety. The governor of the nearby Voronezh region, north of Rostov, also said that a convoy of military equipment was moving along a local highway. It was not clear which direction it was moving.

As the events played out in Russia, Ukraine’s armed forces posted three words on Twitter: “We are watching.”

Who is Prigozhin?

The St. Petersburg tycoon has for years been part of a charmed circle of Russian oligarchs with close ties to President Putin. In 2018, he was one of 13 Russians indicted by a federal grand jury in the United States for interfering in the 2016 American election.

Mr. Prigozhin’s Wagner mercenary force, a shadowy private military company, first emerged during Russia’s illegal annexation of Crimea in 2014. It has since exerted influence on behalf of Moscow in Syria, Libya, the Central African Republic, Sudan, Mali and Mozambique.

Wagner is important to the Kremlin’s war in Ukraine and led the recent assault on the eastern Ukrainian city of Bakhmut. And Mr. Prigozhin, who has recruited fighters from prisons, has been widely seen as a symbol of wartime Russia: ruthless, shameless and lawless.

Why is Prigozhin angry?

Mr. Prigozhin has in recent months launched accusations at Russia’s military leadership. He blames Russian generals for failing to provide his forces with enough ammunition and for ignoring soldiers’ struggles.

The Kremlin tolerated his broadsides for months, even as some analysts said that Mr. Prigozhin was poised to turn his new prominence into broader political influence, possibly threatening Mr. Putin’s grip on power.

But official patience had clearly evaporated by Saturday morning, when the country’s prosecutor general announced that Mr. Prigozhin was being investigated on charges that carried a maximum prison term of 20 years. TASS, a Russian state news agency, reported that he had been charged.

Later on Saturday morning, Mr. Putin addressed the situation in Rostov-on-Don during a brief speech on state television, saying that “decisive actions” would be taken to stabilize it. He said the functioning of military and civilian institutions in the southern Russian city of a million people had “essentially been blocked.”

“Actions that divide our unity are in essence defeatism before one’s own people,” Mr. Putin said.

On Friday, when a long-running feud between the Russian military and the paramilitary forces of Yevgeny V. Prigozhin escalated into what Russian generals have called a “coup attempt,” Mr. Putin was nowhere to be seen.

The Kremlin’s spokesman, Dmitri S. Peskov, reported that the Russian leader met “around the clock” with officials from the military, the Interior Ministry, the National Guard and the Federal Security Service. He was, Mr. Peskov said, issuing “directives in relation to the attempted rebellion.”

But on Saturday, as Mr. Prigozhin claimed that his fighters had taken control of Russia’s southern military headquarters in Rostov-on-Don, Mr. Peskov said the Russian leader would soon address the nation, in a sign of the urgency of the situation.

Mr. Putin’s whereabouts over the course of the day were unclear, despite unverified reports of helicopters whisking him out of the Kremlin.

Additional videos circulating online and verified by The Times show dozens of soldiers getting out of military vehicles and pointing their guns at the compound. The videos were released hours after Mr. Prigozhin claimed Russia had attacked his forces in Ukraine, and said that he was bringing a column of troops to Russia in retaliation.

Mr. Prigozhin on Friday accused the Russian military of attacking his mercenary forces, escalating a long-running feud with the Russian military over the war in Ukraine. Russia’s prosecutor general announced hours later that Mr. Prigozhin was being investigated “on suspicion of organizing an armed rebellion.”

Since then, the scene that has unfolded in this city of more than one million people is tense and opaque; videos shared on Telegram show tanks driving down streets, soldiers jumping off trucks, and soldiers surrounding the headquarters. The soldiers are widely believed to be Wagner forces.

The Southern Military District is the smallest of Russia’s four regional military authorities, but it has played a critical role in the war. Units belonging to the command have fought in Zaporizhzhia and Donetsk Oblasts.

Contributing to the confusion emanating from Rostov-on-Don, civilian onlookers next to the military show of force seem to be unusually calm. In videos circulating online, some people can be seen smoking and drinking beer, and a street cleaner carries on with sweeping litter.

Robin Stein contributed reporting.

Serhiy Popko, the head of the Kyiv military administration, said air defenses had shot down more than 20 missiles around the capital but that falling debris had hit a high-rise building and started a blaze that destroyed three floors. In addition to the dead, 11 people were injured, according to Ukraine’s Interior Ministry. As rescuers worked at the scene, Vitali Klitschko, the mayor of Kyiv, said that there might be people under the rubble.

Just after dawn, smoke was still rising from the building as firefighters used a crane to observe damage to its 16th, 17th and 18th floors. Residents carefully stepped over the broken glass and building fragments that had scattered throughout the parking lot below.

Two women, their legs dotted with small wounds, walked out from the building. One was wrapped in a blanket; the other wore only a robe.

Residents of Kyiv had been shaken from their sleep just before 2:30 a.m. local time as air-raid sirens blared. Mr. Klitschko, reported explosions as other officials said air defenses were working and urged people to take shelter.

Volodymyr and Iryna Kuts were awakened in their apartment on the 19th floor by a crash as debris tore through the stories below.

“I don’t know how we survived,” said Mr. Kuts, 65. Their windows were blown out, and smoke filled the air.

“We were just hugging thinking we would suffocate,” Ms. Kuts, 62, said. They eventually made their way down the stairs, helped by police officers.

Outside the building, dozens of residents milled around, many looking toward the charred and gaping hole ripped into its side.

Dymytro Romanov, 42, lives in a neighboring high-rise and said it was a matter of chance that his building was unscathed.

“I also live on the 18th floor,” Mr. Romanov said as he pointed toward his building. “But I got lucky.”

Emergency workers were still on the scene, helping evacuate the wounded: an older couple, a woman on a stretcher and a man walking on his own, covered in dust and debris. One woman arrived at the scene, saying she had come to look for her son who lived on the 18th floor.

But there was “nothing left” on the 18th floor, a firefighter nearby said after she’d walked away.

Just before 6 a.m. the wail of an air-raid siren pierced the air again, sending residents racing for an underground shelter.

Cassandra Vinograd contributed reporting.

Mr. Prigozhin is a businessman who amassed his wealth partly through securing catering contracts from the Kremlin, resulting in the “chef” moniker. His Wagner mercenary force has exerted influence on behalf of Moscow in Syria, Libya, the Central African Republic, Sudan, Mali and Mozambique, and it is now a crucial force fighting on Russia’s behalf in Ukraine — though Mr. Prigozhin publicly acknowledged his connection to Wagner only last September.

Since then, he has built an aggressive social media presence, portraying himself and his forces as more ruthless and effective fighters than the Russian military, and denouncing Moscow’s defense bureaucracy — all while maintaining a close alliance with President Vladimir V. Putin.

Mr. Prigozhin’s pointed accusations about the competency of the Russian defense ministry, paired with his fighters’ advances in the grinding battle for Bakhmut, transformed him from a once-secretive figure into a political power player on the public stage.

The discord between Mr. Prigozhin and Russian defense officials became more exposed as the first anniversary of the war approached, in February of this year.

At that time, Mr. Prigozhin’s mercenary group was losing its ability to replenish its ranks. His troops’ sheer numbers — bolstered by prison inmates personally recruited by Mr. Prigozhin — had enabled Wagner’s repeated, costly offensives in Bakhmut. But news of Wagner’s astronomical casualty rate was spreading to Russian penal colonies, and Mr. Prigozhin said in early February that he would stop recruiting inmates, without giving a reason.

Not long afterward, he took aim at figures near the very top of Russia’s command structure, accusing the defense minister and the country’s most senior general of treason in vitriolic, profanity-laden audio messages on social media.

Mr. Prigozhin claimed that military officials were deliberately withholding ammunition and supplies from Wagner fighters in Bakhmut to undermine him, while, he said, Russian forces elsewhere faced failure after failure.

According to a classified U.S. intelligence document that was leaked online in April, the dispute grew so bad that Mr. Putin became personally involved, calling Mr. Prigozhin and Russia’s defense minister, Sergei K. Shoigu, into a meeting believed to have taken place on Feb. 22. “The meeting almost certainly concerned, at least in part, Prigozhin’s public accusations and resulting tension with Shoygu,” the document says, using an alternative transliteration of the minister’s name.

The public intensity of the dispute fluctuated over time. Mr. Prigozhin eventually said his fighters in Bakhmut had received the ammunition they needed, and in April, Russia’s defense ministry made a rare acknowledgment of their cooperation, saying that Russian paratrooper units were covering Wagner’s flanks in the western part of the city.

But over the course of three weeks in May, Mr. Prigozhin issued a series of inflammatory statements. He again accused Russia’s military bureaucracy of starving Wagner forces of the ammunition they needed to fully capture Bakhmut, this time threatening to withdraw them from the city on May 10. He appeared to backtrack two days later, as he had done before, this time saying he had received satisfactory promises of more arms. He undermined the Russian Army’s claims of a partial “regrouping” of its forces in the city by declaring it a “rout,” and denied a report that he had offered to betray the Russian Army’s locations around Bakhmut if Kyiv agreed to withdraw from the area. In late May, he declared that Bakhmut was fully under Wagner control.

Kyiv swiftly denied the claim. Several hours later, Russia’s defense ministry released a statement saying that the city’s capture “has been completed” as a result of Wagner’s actions with the support of traditional Russian forces.

Despite the recognition, Mr. Prigozhin soon went back to lambasting Russia’s military leadership — culminating in his allegations on Friday that Moscow’s generals said amounted to a coup.

Here’s what to know about Wagner:

How did Wagner get its start?

The entity first emerged in 2014, during Russia’s annexation of Crimea, and has evolved over the years to become more of a private contractor for Russia’s military. Mr. Prigozhin had been referred to as “Putin’s chef” because of his catering business, which has staged elaborate state banquets for Mr. Putin.

How did the group get its name?

The group reportedly took its name from the nom de guerre of its leader, Dmitry Utkin, a retired Russian military officer. Mr. Utkin is said to have chosen Wagner to honor the composer, who was a favorite of Hitler’s. Despite the Kremlin’s denial of any ties to Wagner, Mr. Utkin has been photographed next to Mr. Putin.

Where is the group based?

The group is not registered as a legal entity anywhere in the world. Mercenaries are illegal under Russian law. Their shadowy existence allows Russia to downplay its battlefield casualties and distance itself from atrocities committed by Wagner fighters, observers say.

“It operates in a situation of opacity, there’s a real lack of transparency and that’s the whole point,” said Sorcha MacLeod, the leader of the United Nations Working Group on the use of mercenaries, which has scrutinized the group. Their structure allows them to have plausible deniability and to create “distance between the Russian state and the group,” she said.

Why are the mercenaries in Ukraine?

Russia has suffered heavy losses in the war and has sent poorly trained recruits to the front lines as cannon fodder. Wagner’s forces led the nearly yearlong assault on the eastern city of Bakhmut. After an estimated tens of thousands of casualties on both sides, Russia declared victory, though there is still fighting in the area.

Where do they recruit?

Wagner’s forces include veteran fighters and mercenaries, and the group had experience fighting in Ukraine’s eastern Donbas region for years before Russia’s full-scale invasion. But the use of convicts, whom Mr. Prigozhin personally recruited from prisons, emerged prominently in the battle for Bakhmut.

U.S. officials have cited battlefield intelligence reports in crediting Russia’s success in Bakhmut in large part Wagner’s willingness to use those prisoners. Ukrainian soldiers, though, have said that by the end of the battle prisoners were less prevalent, with Wagner’s professional fighters playing a bigger role.

Where have Wagner forces been deployed?

In addition to their involvement in Syria, Libya, Central African Republic and Ukraine, Wagner operatives have also fought in Sudan, Mali and Mozambique, exerting Russian influence by proxy, doing the bidding of authoritarian leaders and, at times, seizing oil and gas fields or securing other material interests. They have become more formalized and have started acting more like Western military contractors.

“There’s a trend or pattern around what happens when Wagner is involved in an armed conflict,” Dr. MacLeod said. “The conflict is prolonged, involves heavy weaponry, civilians are impacted in a substantial way, human rights violations and war crimes increase substantially, and there’s no access to justice for victims.”

But until Friday — when Russian generals accused Mr. Prigozhin of mounting a coup — Mr. Putin had not checked Mr. Prigozhin’s online accusations, despite jailing or fining many other critics of the war.

Spewing vulgarities, disregarding the law and displaying loyalty to no one but Mr. Putin, Mr. Prigozhin, a businessman known as “Putin’s chef” because of his catering contracts with the Kremlin and Russian military, has become a symbol of wartime Russia: ruthless, shameless and lawless. It was a significant turnabout for Mr. Prigozhin, who acknowledged only last fall that he had founded Wagner.

In Moscow, he has been dogged by open questions and criticism, with analysts expressing doubts that his recruitment of prisoners and endorsement of extrajudicial executions had broad appeal.

Mr. Prigozhin expanded Wagner’s presence in Ukraine after the Kremlin’s attempt to seize Kyiv, the capital, failed in the initial days of its invasion early last year. The “private military company” was at that point largely active in Syria and Africa, where it operated both on behalf of the Russian government and in the service of Mr. Prigozhin’s own business interests.

He was also active elsewhere. In February 2018, Mr. Prigozhin was one of 13 Russians indicted by a federal grand jury for interfering in the American election through the Internet Research Agency, a troll factory that spread falsehoods and waged information warfare against the United States, in support of the presidential campaign of Donald J. Trump.

The United States imposed sanctions against Mr. Prigozhin in December 2016.

Born in 1961 when St. Petersburg was called Leningrad, Mr. Prigozhin was sent to prison in 1981 for robbery and other crimes, according to Meduza, an online investigative publication.

After serving his nine-year sentence, he opened a hot-dog stand, eventually leading to an entrepreneurial career starting restaurants and convenience stores.

Videos circulating widely on social media showed that military and national guard armored vehicles had been deployed in Moscow and the southern city of Rostov-on-Don, near the front line in Ukraine where Mr. Prigozhin’s fighters had been operating.

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Mr. Prigozhin on Friday accused the Russian military of attacking his Wagner mercenary forces and, in a series of recordings posted to social media, pledged that his fighters would retaliate. Russian authorities, in turn, accused Mr. Prigozhin — whose broadsides against the Russian Defense Ministry had been tolerated by Mr. Putin for months — of trying to foment a revolt.

“This is a stab in the back of the country and the president,” Gen. Vladimir Alekseyev, the deputy head of Russia’s military intelligence agency, said in a video appeal to Mr. Prigozhin’s fighters, urging them to call off any rebellion. “This is a coup.”

Mr. Prigozhin’s Wagner mercenary force has proved pivotal to the Russian war effort in Ukraine, but in recent months, he repeatedly chastised Russia’s top brass for alleged corruption and indifference to regular soldiers’ lives. On Friday night, he took his accusations to a new level, claiming that the Russian military had attacked Wagner encampments, killing “a huge number of fighters.”

“The evil borne by the country’s military leadership must be stopped,” Mr. Prigozhin (pronounced pree-GOH-zhin) said in one of a series of voice recordings posted to the Telegram social network after 9 p.m. Moscow time.

Minutes later, he suggested that his Wagner mercenary force was prepared to go on the offensive against Russia’s own Defense Ministry, saying, “There’s 25,000 of us, and we are going to figure out why chaos is happening in the country.”

He denied that the actions were a “military coup.”

“This is a march for justice,” he said in another audio message on Telegram. “Our actions aren’t interfering with the troops in any way.”

Just past midnight Moscow time, Russia’s prosecutor general announced that Mr. Prigozhin was being investigated “on suspicion of organizing an armed rebellion” and would face as much as 20 years in prison if prosecuted.

The Wagner leader then defiantly took to Telegram again, saying his fighters were approaching the city of Rostov-on-Don and adding: “We are going farther. We will go to the end.”

Mr. Prigozhin’s whereabouts remained unclear, and there was no immediate confirmation that his forces were actually approaching the city.

While President Volodymyr Zelensky of Ukraine had yet to comment as of Friday night, one of his advisers, Mykhailo Podolyak, warned on Twitter that “tumultuous times are coming” for Russia.

White House officials said they were following the events, but would not say much more. “We are monitoring the situation and will be consulting with allies and partners on these developments,” said Adam Hodge, a National Security Council spokesman.

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Mr. Prigozhin, a St. Petersburg restaurateur who leveraged his personal connections with Mr. Putin into lucrative government contracts, gained international prominence after his online “troll factory” interfered in the 2016 American presidential election — and after his Wagner fighters were deployed in Syria and across Africa as a shadow force believed to be fighting for Kremlin interests.

For months the Russian war effort has been hampered by the bitter feud between Mr. Prigozhin and top military leaders, whom he has accused in scathing terms of incompetence in conducting the war. He has asserted that Russia’s top brass have refused to provide Wagner forces with needed ammunition even as they fought alongside the Russian military for control of the Ukrainian city of Bakhmut.

But never before had Mr. Prigozhin accused Russia’s military leaders of attacking his forces, nor asserted in such stark terms that the Kremlin’s stated justification for the war was nonsense.

In a 30-minute video released on Friday, Mr. Prigozhin had described his country’s invasion of Ukraine as a “racket” perpetrated by a corrupt elite chasing money and glory without concern for Russian lives.

He also accused the Russian minister of defense, Sergei K. Shoigu, of orchestrating a deadly attack with missiles and helicopters on camps to the rear of the Russian lines in Ukraine, where his soldiers of fortune were bivouacked. And he accused Mr. Shoigu of overseeing the strikes himself from the town of Rostov-on-Don in southern Russia, near Ukraine.

The mercenary leader’s claims could not be immediately verified. The Russian defense ministry denied the allegations, saying in a statement that the messages Mr. Prigozhin had posted about supposed strikes on Wagner camps “do not correspond to reality.”

Dmitri S. Peskov, the Kremlin’s spokesman, said that Mr. Putin was “aware of all events around Prigozhin,” according to Interfax, a Russian news agency.

Mr. Prigozhin’s accusations created a ripple effect among Russian pro-war activists, who fear that an open conflict between the army and Wagner forces could threaten the Russian front lines during the Ukrainian counteroffensive. In Ukraine, some viewed his statements as more evidence of internal divisions within the Russian war effort.

In an earlier videotaped speech, Mr. Prigozhin did not explicitly impugn Mr. Putin, instead casting him as a leader being misled by his officials. But in dismissing the Kremlin’s narrative that the invasion was a necessity for the Russian nation, Mr. Prigozhin went further than anyone in Russia’s security establishment in publicly challenging the wisdom of the war.

“The war wasn’t needed to return Russian citizens to our bosom, nor to demilitarize or denazify Ukraine,” Mr. Prigozhin said, referring to Mr. Putin’s initial justifications for the war. “The war was needed so that a bunch of animals could simply exult in glory.”

Friday’s diatribes deepened the enigma of Mr. Prigozhin’s ambiguous role in Mr. Putin’s system. His Wagner troops, composed of veteran fighters as well as thousands of convicts whom Mr. Prigozhin personally recruited from Russian prisons, proved key in capturing the Ukrainian city of Bakhmut in May after a monthslong battle.

But, during the battle for Bakhmut, Mr. Prigozhin also emerged as a populist political figure, excoriating Russia’s military leadership for corruption. His angry recordings and videos posted to the Telegram messaging network cast top military and Kremlin officials as unaware and uncaring of the struggles of regular Russian soldiers.

So far, Mr. Putin has not reined in Mr. Prigozhin, even as Mr. Putin’s security forces have jailed or fined thousands of Russians for criticizing the military or opposing the war. Some people who know Mr. Putin have said they believe that he still sees Mr. Prigozhin as a loyal servant applying needed pressure on a sprawling military apparatus. Others theorized that the Kremlin had orchestrated Mr. Prigozhin’s tirades against Mr. Shoigu, the defense minister, to deflect blame from Mr. Putin himself.

But Friday’s statements complicated the picture, with Mr. Prigozhin going after not just Mr. Shoigu but also unnamed “oligarchs” around Mr. Putin, while casting the entire official rhetoric around the invasion as a sham. He said there was “nothing out of the ordinary” in Ukraine’s military posture on the eve of the February 2022 invasion — challenging the Kremlin’s justification that Ukraine was on the verge of attacking Russian-backed separatist territory in Ukraine’s east.

“Our holy war with those who offend the Russian people, with those who are trying to humiliate them, has turned into a racket,” he said.

Mr. Prigozhin also asserted in his video that Ukraine’s counteroffensive to gain back territory was going much more poorly for Russia than the government was letting on. On Telegram, pro-war commentators quickly pushed back against that claim, including Igor Girkin, a former paramilitary commander who himself has often criticized Russia’s top brass.

“Prigozhin already should have been handed over to a military tribunal for many things,” Mr. Girkin wrote. “Now also for treason.”

Julian E. Barnes and Cassandra Vinograd contributed reporting.