"Trump has no chance," said Kennedy of the Republican candidate in next month's US presidential election.
But the fact that he was even in the running for the White House was indicative of "the rise of the far right all around the world," he said in an interview at his home in Paris.
"It's not just a US phenomenon," he said, pointing to what he called the "Brexit disaster", when Britain voted in a referendum in June to pull out of the European Union.
"Compared to Trump, (former US president) George W. Bush was Chateaubriand," he quipped, referring to the French 18th century writer and historian.
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But he warned: "If he was elected, Trump would be the American Mussolini."
Kennedy, whose 12 novels include "The Big Picture" and "The Pursuit of Happiness", said his vote would go to the Democrat nominee Hillary Clinton.
"She will be a very great president," he said, denouncing what he said was the misogynistic abuse she had faced.
Kennedy, 61, has sold millions of copies of his books worldwide, which have been translated into 22 languages. Several have been adapted for the cinema, including "The Woman in the Fifth" which came out in 2011, starring Ethan Hawke.
He divides his time between Britain, Canada, the United States -- and France, where he has a large following.
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