Sadiq Khan has urged Theresa May to stand up to Donald Trump, as he accused the US president of "playground" behaviour.
The London mayor, who was the subject of an insult in a tweet sent by Mr Trump minutes before he landed for his UK state visit, also defended Jeremy Corbyn's decision to boycott a dinner hosted by the Queen for the American leader.
Mr Khan said he believes Mr Trump has become a "poster boy" for the far-right.
Speaking during a visit to a museum in south London, Mr Khan said: "I think the reality is, when you look around the world there are many leaders whose views I find abhorrent – in Hungary, in Italy, in France, here in the UK, and the poster boy they look up to is Donald Trump.
"And it should be a source of concern to us that Donald Trump is the poster boy for the far-right movement around the world and actually some of the things that he has done as president are deeply objectionable, far more objectionable than silly tweets he might send."
Mr Trump branded the Labour mayor a "stone cold loser" in a two-part tweet as he touched down in Britain on Monday.
Asked how it felt to be described in such a way, Mr Khan said: "I'm not 12 years old in a playground, I'm just surprised Donald Trump thinks he is.
"This is not about childish behaviour, this is about us as a city and many people around the country making clear our views about some of the things said and done by Donald Trump."
Mr Khan said he hoped the Prime Minister would challenge Mr Trump on some of his more controversial behaviour as president, including language around women and minorities and views on climate change.
He said: "These are big issues and I'm hoping that Theresa May stands up for our country and does say boo to a goose."
Mr Khan insisted he is "very pro-America" and likened the US to a "best friend" of the UK's, but stood behind his party leader's decision not to be part of the state visit.
Mr Corbyn attended a state dinner for Chinese President Xi Jinping in 2015, despite protests around that leader's visit concerning the country's human rights record.
Asked if Mr Corbyn's behaviour was hypocritical, the mayor said: "My views are quite clear and I think Jeremy Corbyn's are not dissimilar, which is yes, we should have a close relationship with the president of the USA, yes he should be able to come here on a working visit.
"What shouldn't happen is a state banquet and a state visit with the red carpet rolled out. I think it's inappropriate. I think those visits should be reserved for leaders who have done something and deserve that. I think it sends the wrong message to be seen to condone some of the things this president has said and done."