Sadiq Khan attacked Donald Trump and accused the presidential candidate of "playing into the hands of ISIS" by alienating Muslims.
Speaking whilst on a five-day Canada and America tour, the Mayor of London told audiences during a Global Progress event in Montreal, Mr Khan damned the republican nominee's "lack of understanding".
He said: "Donald Trump said that Muslims from around the world - I'm paraphrasing - would not be welcome into the United States of America.
"Not only does that show a lack of understanding and awareness of the great country that is the USA and its history and legacy, it's also inadvertently playing into the narrative and hands of Daesh and so-called ISIS because it implies it’s not possible to be a Western liberal and mainstream Muslim."
He added: "I think it’s important that the USA maintains her role as a beacon for tolerance, respect and diversity."
Mr Khan shared a platform with Canadian Prime Minister Justin Trudeau after stops at New York City and Chicago to "strengthen ties with other cities" in wake of the Brexit vote.
Both leaders were quick to thank one another for the success of the visit on social media following the event.
While in the United States the Mayor was also joined by 30 of London’s fastest growing technology start-up businesses on a trade mission to help them secure new investment.
His whirlwind trip also included meeting faith groups in Chicago and signing a tech treaty with Chicago Mayor Rahm Emanuel to show that his campaign #LondonisOpen includes the technology industry.