The billionaire said he decided to call off the rally after consulting with police in Chicago, where tensions had been rising for hours in the build-up to the event at a sporting arena at the University of Illinois.
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"I don't want to see anybody hurt," Trump told CNN afterwards. "I think we made the right decision (to cancel)... even though our freedom of speech was violated."
Hundreds of protesters, many of them blacks and Latinos angered by Trump's incendiary anti-immigrant rhetoric, had massed outside and inside the venue itself, mingling with the candidate's supporters.
CNN estimated there were between 8,500 to 10,000 people in the arena when tensions erupted into chaos.
According to an AFP photographer at the scene, scuffles broke out as the decision to cancel was announced, with police struggling to separate angry supporters and protesters.
News footage showed police wearing body armor escorting groups of people out of the building. Police reportedly said there were no arrests and no injuries.
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"We are not rapists," read one sign held by a protester inside the arena, referring to Trump's characterisation last year of Mexicans rapists.
Critics have accused Trump of fuelling the toxic atmosphere. On February 1, as protesters interrupted a rally in Iowa, he encouraged supporters to "knock the crap out of them," and pledged to pay their legal fees.
When a protester disrupted Trump's speech in Las Vegas, the brash billionaire said he would like to "punch him in the face."
Trump dismissed the notion that he was responsible for whipping up tensions.
In a statement, Trump's campaign said he had determined that "for the safety of all of the tens of thousands of people that have gathered in and around the arena, tonight's rally will be postponed to another date."
"Please go in peace," it added.
Pundits said the chaos at the cancelled Trump rally was reminiscent of violent protests at the 1968 Democratic national convention, also in Chicago, held when the United States was torn apart by opposing views on the Vietnam War.
The ugly scenes come just days before the states of Florida, Ohio, Illinois and Missouri vote in the party primaries on March 15, with Trump looking to tighten his grip on the Republican nomination.
The sudden security concerns mark a major test for Trump as he seeks to lock up the nomination and turn his attention to doing battle against Democratic frontrunner Hillary Clinton.
After rattling the Republican establishment with his shock emergence as the man to beat, Trump has been working in recent days hard to look presidential and shake off the brash, belligerent image that has defined his campaign so far.
But Trump's rallies are known for being rambunctious affairs, and that seeped over into violence on Wednesday night in North Carolina when a 78-year-old white man in a cowboy hat punched a black protester in the face.
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John McGraw, who later said that next time "we might have to kill him," was charged with assault, battery and disorderly conduct for the sucker-punch.
The incident drew a sharp rebuke from Clinton and Democratic rival Bernie Sanders, but Trump blamed protesters for stirring trouble and pointed the finger at media for giving disproportionate air time to the disruption.
Asked on CNN, after calm was restored in Chicago, whether he regretted his comments about protesters, or bore some responsibility for whipping up tensions, he refuted the notion.
"No, I don't regret it at all," he said. "Overall I think we've been very mild with protesters."
Trump, who is scheduled to hold a rally Saturday in Cleveland, Ohio, has called on Republicans to amass behind him to propel him into the White House.
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On Friday he received a major boost, winning the endorsement of Ben Carson, the retired neurosurgeon who quit the presidential race last week.
The real estate mogul closed a debate Thursday in Florida with rivals Ted Cruz, John Kasich and Marco Rubio by calling for the party to unite behind him to win the November general election, and doubled down on the theme on Friday.
"The Republican Party should grab this, and we will have a victory like the Republican Party has never had before," he said.
Many in the party see next Tuesday's votes as the last best chance to derail the insurgent candidacy of the billionaire mogul, who has so far won 15 of 24 primary races.
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