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. Fourteen people were killed.
London’s Metropolitan Police issued a rare rebuke after media reported comments by Trump that parts of the British capital were now “so radicalised that the police are afraid for their own lives”.
“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,” said a police spokeswoman.
“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.”
The city’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.