Meta CEO Mark Zuckerberg, Amazon CEO Jeff Bezos and Tesla CEO Elon Musk are all in St. John's with the president-elect.
Their attendance was confirmed by reporters inside the church as well as a photo tweeted by Rep. Mike Waltz.
Former British Prime Minister Boris Johnson is also attending the service.
Trump's second swearing in is occurring on Martin Luther King Jr. Day.
The last time that happened was former President Barack Obama's second inauguration in 2013. Obama, the nation's first Black president, took the oath of office that year using two Bibles, one that belonged to King and one that belonged to Abraham Lincoln.
The overlap isn't expected to happen again until 2053.
Following Biden’s announcement of preemptive pardons to potential targets of the incoming Trump administration, the recipients express their gratitude to the outgoing president.
"I really truly appreciate the action President Biden has taken today on my behalf," Dr. Anthony Fauci told ABC News Chief Washington Correspondent Jonathan Karl. "Let me be perfectly clear, Jon, I have committed no crime, you know that, and there are no possible grounds for any allegation or threat of criminal investigation or prosecution of me.”
Retired Gen. Mark Milley also expressed that he was “deeply grateful for the President’s action.”
"After forty-three years of faithful service in uniform to our Nation, protecting and defending the Constitution, I do not wish to spend whatever remaining time the Lord grants me fighting those who unjustly might seek retribution for perceived slights. I do not want to put my family, my friends, and those with whom I served through the resulting distraction, expense, and anxiety."
"I wish this pardon weren't necessary, but unfortunately, the political climate we are in now has made the need for one somewhat of a reality,” said Harry Dunn, former Capitol Police officer. “I, like all of the other public servants, was just doing my job and upholding my oath, and I will always honor that.”
Donald Trump and Melania Trump have arrived for a church service at St. John's in Washington, a tradition for presidential inaugurations.
Trump said he's feeling "great."
After church, President Biden and first lady Jill Biden will welcome the Trumps to the White House -- an invitation that Trump did not offer the Bidens four years ago when he skipped the inaugural events entirely.