"Some represented Bernie, our Communist friend," the controversial real-estate tycoon alleged in Dayton, Ohio. He repeated this at his second rally in Cleveland. "You know Bernie was saying Trump should speak to his crowd. You know where they come from? Bernie's crowd. They're Bernie's crowd," Trump said.

The allegations by Trump, 69, was immediately denied by the Sanders campaign, which disassociated with the protesters who were chanting "Bernie Bernie" at Trump's Chicago rally.

The Chicago rally was cancelled by Trump, a quite unusual development in America's electoral history. "Get your people in line, Bernie," Trump said at his Cleveland rally, as he launched a frontal attack on Sanders.

Sanders, the 74-year-old Democratic Senator from Vermont, who is running his presidential campaign on the slogan of democratic socialism, was quick to fire back.

"As is the case virtually every day, Donald Trump is showing the American people that he is a pathological liar. Obviously, while I appreciate that we had supporters at Trump's rally in Chicago, our campaign did not organise the protests," Sanders said in a statement.

"What caused the protests at Trump's rally is a candidate that has promoted hatred and division against Latinos, Muslims, women, and people with disabilities, and his birther attacks against the legitimacy of President Obama," he said.

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