The demonstration follows US President Donald Trump's cancellation of a planned visit to the United Kingdom capital next month.
The chaos lasted around 20 minutes - during which Mr Khan read the newspaper and a rival protest of anti-Brexit supporters was cheered by the crowd - as the police were called in to deal with the disruption.
"Making his reflection on the London's mayors increasing concern with matters of state instead of effectively running the city he was elected to lead, Kassam concluded: "...to borrow a phrase from Donald Trump, London is turning into a shithole under Sadiq Khan".
Shortly after Mr Khan began speaking at the society's new year conference, one man told the audience: "Ladies and gentlemen, we're here today to make a non-violent, peaceful citizen's arrest".
But a demonstrator, unperturbed by the reception, said: "We stand under common law jurisdiction".
"If you touch us, you'll be done for common assault".
It comes after the London Mayor said Trump had "got the message" that he isn't welcome in the capital after he cancelled his visit to the new American embassy. The protesters were later escorted from the venue by police.
"We're not leaving, we've paid for a ticket".
Footage appeared to show the moment the activists interrupted Mr Khan's speech, just as the mayor was introduced to the stage.
He then accused the Fabian Society of "subverting" the constitution.
Last Saturday the U.S. president used the same phrase to describe himself in a tweet following questions about his mental health.
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".