Sadiq Khan has told Donald Trump he should expect loud protests if he visits London this year.
The London mayor, who has repeatedly clashed with the US president, made the remark following reports that Mr Trump has a UK visit in the pipeline.
Mr Khan said at a St George's Day event in Trafalgar Square the capital: "We have got a great history in our city of protests... we have got a great history in our city of bringing about change by protest, the key thing is for it to be lawful, for it to be peaceful.
"I have no doubt that if he does come, there will be some people who want to express their views loudly and peacefully to the president."
Mr Khan said if the visit does go ahead, Mr Trump will see a city where those from different backgrounds, religions and ethnicities get on very well with each other.
He said that "the message should be loud and clear that we think it is wrong that anyone should be amplifying far-right messages" or messages of hatred or division.
He added: "I was very disappointed to be honest that the leader of our closest ally was doing just that, and we shouldn't be embarrassed to say to our best friend that we think they are wrong.
"I think our best friend, the USA, their leader was wrong when he retweeted messages from a Far Right group."
Last November Mr Trump came under fire for retweeting a number of inflammatory videos posted by Britain First deputy leader Jayda Fransen – prompting Prime Minister Theresa May to condemn him at the time.
Mr Trump has not visited the UK since he was elected to the White House in November 2016, with a potential visit to London to open the US Embassy in January not taking place.
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The president blamed the cost of the new embassy and its location south of the River Thames, saying it was a "bad deal".
The Daily Mail has reported a visit from Mr Trump has been "pencilled in" for July, while the Daily Telegraph quoted an unnamed US official saying the president was looking at coming in the "late summer".
Additional reporting by Press Association
