The group failed, and was eventually escorted out by the same police officers they had asked to apprehend Mayor Sadiq Khan. They still managed to delay the speech for 15 minutes as they accused Khan of treachery, tyranny and disrespecting President Donald Trump.
Trump - who once sought to ban Muslims from U.S. shores and called Khan "pathetic" after a terrorist attack in London last year - earlier this week abruptly canceled plans to visit the city. Khan was one of many Britons who welcomed the news, claiming Trump had been scared off by threats of mass protests.
The mayor was scheduled to give a speech at a left-leaning think tank, the Fabian Society, on Saturday. In the mayor's own words, it was about "about how we cannot allow a populist, anti-feminist narrative to take hold in Britain."
But as Khan prepared to give his address, a man named Davey Russell and about half a dozen others towed a wooden frame with a noose onto a street outside the auditorium. They had decorated it with the emblem of a white dragon, and written on the top: "TAKE BACK CONTROL."
Russell told reporters outside the conference that his group was called the Pendragons. But an ITV reporter, among others, identified him as a leading member of the English Defense League, a well-known far-right organization. Russell also hosts an anti-Islamic radio show, which Kent Online reported was raided by police several years ago.
On Saturday, Russell told reporters that he believed he had the law on his side. He accused Khan of cooperating with terrorists and placing himself above the law, and said the mayor had no right to criticize Trump.
"He should keep his nose out of state affairs and look out for what's going on in London," Russell said.
Carrying a piece of paper on which he had outlined the mayor's alleged crimes, Russell walked with his Pendragons to the front of the auditorium and began to live stream.
Khan took the microphone and managed to say, "Thank you," before a man in a trench coat approached the table and interrupted his speech.
"Ladies and gentlemen, we are here today to make a peaceful, nonviolent citizen's arrest of those named," the man said. He named Khan and Parliament member Kate Green, who was chairing the event, and two other speakers.
Khan took a drink of water, sat down and looked at the men. He said nothing.
"Could I ask security to lead the gentlemen out, please?" said Green.
A guard walked over to Russell, who seemed to have been expecting a challenge.
"We are under common law jurisdiction!" Russell cried. "If you touch us, you will be done for common assault. We're not leaving."
But the guard persisted, and soon Russell was arguing: "I paid for a ticket! Do not touch me," he said. "We have paid for a ticket. . . . OK, so we want the money back for the ticket then. We cannot be reimbursed for the ticket. We cannot be reimbursed!"
He said he had called the police before crashing the speech, and promised they would soon arrive to arrest the mayor. It would just be a few minutes.
"Please don't panic, anybody," Russell said.
The audience started to slow clap.
"Perhaps you could wait outside?" Green said after a few minutes. But the Pendragons didn't want to wait outside.
A small, mostly curious crowd formed around the men as they lingered by the auditorium wall, accusing Khan of subverting British law, without ever explaining how. When a reporter asked under what authority they planned to arrest the mayor, one of the Pendragons cited the Magna Carta.
Police had still not arrived after 10 minutes or so. One of the Pendragons walked up to Khan's table and showed him an American flag, which he was holding backward.
"Mr. Khan," the man said, "there's millions of British people supporting Donald Trump."
The audience booed especially loudly at this. "Good one!" Russell cheered.
"Please sit down, sir," Green said. So the man walked back to Russell, holding his flag up to the news cameras, still holding it the wrong way.
Green eventually began to lose patience with the men. "I am not having you dictate the way this meeting is conducted," she said. Eventually, Sky News reported, Khan began to read a newspaper.
After a quarter-hour standoff, Russell pointed his cellphone at his own face and happily announced to his Facebook viewers: "There's an inspector outside we have to speak to now over these charges against Sadiq Khan."
But when the constable walked into the room, he simply told Russell: "You'll have to leave now."
Russell handed the officer a sheet of paper outlining his complaints. One of them cited the Coronation Oath Act of 1688. The officer looked at it. "We are going to investigate," he promised, not entirely convincingly.
"Well, the grounds for the arrest are all there," Russell said.
"We don't do citizen's arrests," the officer explained. "We do lawful arrests."
Russell finally followed the constable outside, to much applause from the crowd. One of his compatriots waved a water bottle aloft as they left.
Khan stood up and resumed his speech. "It's a pleasure to be here," he said, "even though we were distracted by the actions of what some would call very stable geniuses."
The crowd laughed at this. Outside in the lobby, his gallows still empty, Russell continued to argue with police. He also asked for a refund on his ticket.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)