As London Mayor Sadiq Khan prepared to take the stage at the yearly Fabian Society conference on Saturday, he was interrupted by American flag-holding Trump supporters who attempted to stage a "citizens arrest".
Khan was set to begin a speech on gender equality at the left-leaning think tank the Fabian Society when a group called the White Pendragons called for Khan's arrest as well as shouting pro-Trump and Brexit slogans.
When Khan, a former chair of the Fabian Society, resumed his speech he quipped: "On the plus side, I wasn't served with a P45" - a reference to Theresa May's Tory conference speech that was interrupted by a prankster. That didn't pan out.
Police officers then moved in to escort them from the venue.
This comes amid a public war of words between the two leaders where Khan has in the past accused Trump of possessing "ignorant views about Islam" and Trump has jibed at the mayor's response to London terror attacks.
Audience members began to heckle and slow-clap, and security guards entered the auditorium.More news: Keith Jackson, Legendary 'Voice of College Football,' Dies at 89
The incident occurred after Khan said the U.S. president "finally got the message" that he is "not welcome" in the United Kingdom after Trump announced he will not be visiting in February to open the new USA embassy as planned.
But a demonstrator, unperturbed by the reception, said: "We stand under common law jurisdiction".
Mr Trump previously described himself as a "very stable genius" after his mental ability was questioned.
"If you touch us, you'll be done for common assault".
"We're not leaving, we've paid for a ticket".
Trump said he abandoned his plans to visit the United Kingdom as he is "not a big fan of the Obama Administration" selling off the previous embassy and building a new one in an "off-location".