Speaking at the event in Brighton, Khan was asked why he thought the US president chose to tweet about him so often.

Trump has tweeted his disapproval of the London mayor several times, most recently earlier this month when he called him "The incompetent Mayor of London, Sadiq Khan".

Khan said while the US is one of the nation's closest allies, he has no idea why "Donald Trump tweets about the Muslim mayor of London".

"The serious point is this. Do we stand up for our values, and do we, you know, take an argument against somebody who in what he says and what he does are the opposite of our values?" he said.

"Or do keep quiet? And the thing about America is we do have a special relationship with America.

"They are our closest allies and of course that means at times of adversity we stand shoulder to shoulder with them, but Donald Trump is the poster boy for the far-right around the world.

"This is a man who retweets the tweets of known racists and amplifies the messages of known fascists. This is a man who equates anti-racist marches in Charlottesville with anti-Semites and white supremacists."

He said at the time of his election in 2016, when "according to political commentators in London we had the most racist, divisive, Islamophobic campaign in history, Londoners' response was to choose hope over fear, unity over division and love over hatred".

Sadiq is the son of Muslim immigrants.

Australian Associated Press