On Tuesday, Donald Trump, the billionaire businessman and Republican presidential front-runner, followed up his incendiary call to halt Muslim arrivals in the United States by pointing to the supposedly dire security situation in European capitals.

"Paris is no longer the safe city it was. They have sections in Paris that are radicalized, where the police refuse to go there. They’re petrified. The police refuse to go in there," he said on MSNBC.

Then he looked across the English Channel: "We have places in London and other places that are so radicalized that the police are afraid for their own lives. We have to be very smart and very vigilant."

These comments, like Trump's anti-Muslim declaration yesterday, led to a swift, stern response.

London's Metropolitan Police weighed in, rejecting the suggestion that there were areas in the city that officers were too scared to patrol.

"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," a spokeswoman for the police said in a statement.

She offered Trump a tutorial: "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."

London's mayor, the conservative politician Boris Johnson, also chimed in.

"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 radicalization is simply ridiculous," he said.

"The only reason I wouldn’t go to some parts of New York is the real risk of meeting Donald Trump," Johnson quipped.

Sadiq Khan, a member of Parliament from the Labour Party who is seen as a future London mayoral candidate, had his own choice words.

Trump "should apologize for pretending to speak on behalf of our police, who do such an incredible job keeping our city safe," Khan said. "Trump can’t just be dismissed as a buffoon — his comments are outrageous, divisive and dangerous — I condemn them utterly and hope his campaign dies a death."

Others on social media took a lighter view of Trump's scare-mongering rhetoric.

The #TrumpFacts meme echoes the earlier #FoxFacts hashtag, which trended earlier this year in Britain after an "analyst" on Fox News suggested that the city of Birmingham, the second largest in Britain, was "totally Muslim" and unsafe for non-Muslims. The laughable claim was widely mocked.

Similarly on Tuesday, Trump's remarks led to a flurry of droll tweets. The deputy mayor of Paris tagged Trump with a photo showing the city's perilous working-class neighborhoods.

A statement from Paris city hall denounced Trump's rhetoric, insisting that bigotry was not needed to guarantee safety: "While terrorism knows no borders, hitting France just like the US, that has in no way taken away the fact that Paris is a safe and welcoming city.”

J.K. Rowling, author of the beloved Harry Potter books, delivered perhaps the most withering rebuke, likening the Republican front-runner to the arch-villain of her stories — which happen to pit a xenophobic cabal of sorcerers against a more inclusive, tolerant faction.

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