A caller to James O'Brien's LBC radio show hung up after being accused of lying.
The man, called Daniel, had fabricated London Mayor Sadiq Khan's response to a video of Tory MP Mark Field grabbing a protester by the throat as he escorted her out of Philip Hammond's Mansion House speech on Thursday evening.
Mr Field has been suspended as a minister while investigations take place after he physically removed a Greenpeace protester from the black tie event.
Mr Khan responded to the video describing Mr Field's actions as "truly shocking" and said he welcomes the City of London Police's investigation. But Daniel told O'Brien the mayor had "undermined" the security of politicians.
"In the situation of Mark Field, Sadiq Khan and the press are undermining the security of our politicians," he claimed.
But asked how the mayor had undermined the safety of politicians, he replied: "He's saying it's perfectly acceptable for people to intrude..."
O'Brien interjected: "He hasn't said that, now you're actually lying. When did he say it was 'perfectly acceptable' for people to march into private functions and protest?"
The call came to an abrupt end when Daniel hung up after being called out by the radio show host.