The prime minister has joined the mayor of London in rejecting comments by US presidential hopeful Donald Trump that parts of London are "so radicalised the police are afraid for their lives".
The Republican nomination contender made the remarks in a TV interview.
A Downing Street spokeswoman said David Cameron "completely disagrees" with Mr Trump and thinks "his views are wrong".
The mayor of London Boris Johnson said Mr Trump's comments were "complete and utter nonsense".
"The Prime Minister completely disagrees with the comments made by Donald Trump, which are divisive, unhelpful and quite simply wrong," his official spokeswoman said.
The comments represent a departure from the practice usually followed by British prime ministers of avoiding commentary on the merits of contenders in the US presidential race.
Opposition leader Jeremy Corbyn said the presidential hopeful's remarks were an "affront to common humanity" and called for people to "unite against racism", coming in the wake of Mr Trump's call to ban Muslims from entering the US.
The Metropolitan Police 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."
'Outrageous and dangerous'
Mayor Boris Johnson said the remarks were ill-informed.
"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," he said.
Mr Johnson said police officers were doing "excellent work" in London.
"Crime has been falling steadily both in London and in New York - the only reason I wouldn't go to some parts of New York is the real risk of meeting Donald Trump," he added.
Fiyaz Mughal, director of Tell Mama, an organisation that monitors Islamophobic attacks, said: "He plays up to what groups like IS want - that is divided and weaker communities."
Mr Trump's comments risked marginalising the people who could help fight extremism, said Mr Mughal.
The Republican party frontrunner was also attacked by Labour candidate for London mayor Sadiq Khan, who said the billionaire "can't just be dismissed as a buffoon - his comments are outrageous, divisive and dangerous".
Conservative mayor candidate Zac Goldsmith said Mr Trump was "an appalling creature... one of the most malignant figures in politics".
Asked on MSNBC if Muslims were needed in the effort to help confront terrorism, Mr Trump said: "If you look at Paris - and I hate to do this, the chamber of commerce is gonna go crazy - Paris is no longer the same city it was.
"They have sections in Paris that are radicalised - police refuse to go in there. They're petrified.
"We have places in London and other places that are so radicalised the police are afraid for their own lives."
The BBC's Anthony Zurcher, in Washington, says Mr Trump's poll ratings have risen after other hardline statements.
In January, US broadcaster Fox News was forced to issue an on-air apology for televising a claim that Birmingham was a "Muslim-only city" where non-Muslims "don't go".