Sadiq Khan has branded Donald Trump "racist, sexist, misogynistic" and "Islamophobic" following the US president's latest attack on London.
The London mayor said he was "living rent free" in Mr Trump's head after the US president used his address to the UN General Assembly on Tuesday to criticise the UK, and London specifically.
The president continued his long-running criticism of Mr Khan and told delegates the UK capital was being run by a "terrible mayor" who had "changed" the city.
One of his most outlandish claims was that Sharia law, Islam's legal system, was operating in London, which there is no evidence for.
The president said: "I look at London, where you have a terrible mayor, terrible, terrible mayor, and it's been changed, it's been so changed.
"Now they want to go to Sharia law. But you are in a different country, you can't do that."
Asked about the attacks made by the US president, who enjoyed his second state visit to the UK last week, Sir Sadiq said: "I think President Trump has shown he is racist, he is sexist, he is misogynistic, and he is Islamophobic."
He went on to say the public would be "wondering what it is about this Muslim mayor who leads a liberal, multicultural, progressive, successful city that means I appear to be living rent-free inside Donald Trump's head".
"When people say things, when people act in a certain way, when people behave in a certain way, you've got to believe them," he added.
Mr Trump has also blamed crime in London - which he said was "through the roof" - on Sir Sadiq's leadership, and said he had also been a "disaster" on immigration.
The president's comments, which he made on Air Force One on the way back to Washington from London after the state visit, prompted a spokesperson for the mayor to issue a staunch defence of the city.
"London is the greatest city in the world, safer than major US cities, and we're delighted to welcome the record number of US citizens moving here," they said.
Sir Sadiq and Mr Trump have long had a thorny relationship, dating back to the president's call in 2015 for a "total and complete shutdown" of Muslims entering the US.