US presidential hopeful Donald Trump has been mocked by Boris Johnson and criticised by Downing Street after claiming that parts of London were "so radicalised" that police were "afraid for their own lives".
David Cameron's official spokeswoman said the Prime Minister would "clearly disagree" with the would-be president and think that his views are "wrong", while the London mayor said Mr Trump's "ill-informed" comments were "nonsense".
Mr Trump's latest outspoken statements followed his call for Muslims to be barred from entering America, which were slammed as "divisive, unhelpful and quite simply wrong" by Mr Cameron.
Speaking to MSNBC in the US, Mr Trump said: "We have places in London and other places that are so radicalised that the police are afraid for their own lives. We have to be very smart and very vigilant."
Mr Cameron's official spokeswoman said: "The Prime Minister would clearly disagree with Donald Trump and think that his views are wrong."
Mr Johnson ridiculed the business tycoon, who is seeking the Republican nomination for next year's presidential election, saying "the only reason I wouldn't go to some parts of New York is the real risk of meeting Donald Trump".
He said: "Donald Trump's ill-informed comments are complete and utter nonsense.
"As a city where more than 300 languages are spoken, London has a proud history of tolerance and diversity and to suggest there are areas where police officers cannot go because of radicalisation is simply ridiculous.
"I would welcome the opportunity to show Mr Trump first hand some of the excellent work our police officers do every day in local neighbourhoods throughout our city.
"Crime has been falling steadily both in London and in New York - and the only reason I wouldn't go to some parts of New York is the real risk of meeting Donald Trump."
A Metropolitan Police spokeswoman said: "We would not normally dignify such comments with a response, however on this occasion we think it's important to state to Londoners that Mr Trump could not be more wrong.
"Any candidate for the presidential election in the United States of America is welcome to receive a briefing from the Met Police on the reality of policing London."
Mr Trump's call for a "total and complete shutdown" of Muslims entering the US came in the wake of last week's terror attack in San Bernardino, California, where a Muslim couple believed to have been radicalised shot 14 dead at a health centre.
Speaking to a regular media briefing in Westminster, Mr Cameron's official spokeswoman declined to say whether Mr Trump's comments might lead to him being barred from entry to the UK under hate-speech laws.
But she said: "The Prime Minister completely disagrees with the comments made by Donald Trump, which are divisive, unhelpful and quite simply wrong."
The Downing Street statement represents a departure from the practice usually followed by British prime ministers of avoiding commentary on the merits of contenders in the US presidential race.
MP Zac Goldsmith - the Conservative candidate for the London mayoral election - said it was time the US realised that Mr Trump was not a joke but "one of the most malignant figures in modern politics".
"Donald Trump began as a bit of a joke - there are parallels here by the way," he told journalists at a Westminster lunch - in a dig at the unexpected victory of Jeremy Corbyn in Labour's leadership contest.
"People probably quite liked the idea of him ruffling feathers and knocking people around a bit during the campaign for selection.
"But he has gone way beyond being a joke now and he has become - as he clearly always was but has become overtly - an utterly repellent figure in modern politics.
"I just pray that when America gets off this funfair ride they are on at the moment, they realise that Donald Trump is one of the most malignant figures in modern politics and I hope his campaign ends in absolute disaster."
The idea of barring all Muslims, including those from the UK and other Western nations, was "completely insane", he said - but even more insidious was his demand that Muslim US citizens should have to register.
"I don't think he went so far as to say they should wear something on their lapel, but he might as well have done," he said in a reference to the Nazis' treatment of Jews in Germany.
Campaigners against Mr Trump's golf course development in Scotland have submitted a petition to Parliament calling for him to be barred from the UK "for his continued, unrepentant hate speech and unacceptable behaviour". The petition is under consideration for inclusion on the parliamentary website.
Mr Cameron's spokeswoman declined to say whether the PM would be willing to meet Mr Trump or whether he might be blocked from entry to the UK, describing the questions as "hypothetical".
She added: "The Prime Minister has been very clear that, as we look at how we tackle extremism and this poisonous ideology, what politicians need to do is look at ways they can bring communities together and make clear that these terrorists are not representative of Islam and indeed what they are doing is a perversion of Islam."