LONDON (AFP) - The British government and "Harry Potter" author J.K. Rowling on Tuesday condemned comments by US presidential hopeful Donald Trump saying Muslims should be barred from entering the United States.

Rowling wrote on Twitter that "Voldemort was nowhere near as bad" as the Republican frontrunner, referring to the villain of the "Harry Potter" series.

Meanwhile, British Prime Minister David Cameron "completely disagrees" with Trump's remarks, which are "divisive, unhelpful and quite simply wrong", his spokeswoman said.

Trump called for a "total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what is going on".

His comments came after a mass shooting in California by a Muslim couple who were believed to have been radicalised that left 14 people dead.

After British media reported comments by Trump that parts of London were now "so radicalised that the police are afraid for their own lives", the capital's Mayor Boris Johnson said Trump was talking "complete and utter nonsense".

"To suggest there are areas where police officers cannot go because of radicalisation is simply ridiculous," Johnson said in a statement.

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

A petition has also been launched to the British parliament to "Block Donald J Trump from UK entry".

Trump's rivals in the Republican presidential field quickly attacked the statement, the latest in a series of increasingly virulent remarks in recent weeks, and the White House described it as "totally contrary" to US values.

Despite broad public outrage at many of his remarks, the billionaire real estate mogul is ahead in polls of likely Republican voters and is the clear frontrunner to secure the Republican nomination as presidential candidate in 2016.