Fighting erupted between supporters and opponents of Donald Trump after the Republican presidential front-runner scrapped a campaign rally in Chicago due to security concerns.

Throughout the afternoon thousands of anti-Trump protesters had gathered inside and outside the arena at the University of Illinois at Chicago, prompting the billionaire businessman to announce in a statement that he was postponing the event for the "safety" of those in attendance.

As the news reached the protesters inside the packed 10,000-seat venue they began chanting and cheering, "We stopped Trump."

Several fistfights broke out between the competing groups, leading to the arrests of five participants and injuries to two police officers, interim police Superintendent John Escalante told the media.

Trump's statement said the decision to cancel the rally had been made after talks with Chicago police, but Escalante said his officers were never consulted "and we had no role in whether or not the event should be canceled," the Chicago Tribune newspaper reported.

In an interview afterwards with CNN, Trump said he had no intention of causing any violence.

"I don't want to see anybody getting hurt," he said. "When you have thousands of people, you don't want to see a clash."

He described the violence as "minor skirmishes," which reflected divisions in the country over the lack of wage increases, job instability and worry over the economy.

Opponents seized on the incident to slam Trump.

Texas Senator Ted Cruz, Trump's nearest rival, said the protesters were ultimately to blame for the brawls.

"But in any campaign, responsibility starts at the top," he told reporters in Illinois, which holds its Republican primary next week.

"When the candidate urges supporters to engage in physical violence, to punch people in the face, the predictable consequence of that is that it escalates," Cruz said.

Trump has been criticized for not forcefully condemning earlier outbreaks of fighting at his rallies, and in some instances has seemed to suggest violence against protesters could be justified.

Senator Marco Rubio, whose campaign has struggled to pick up momentum in recent weeks, told Fox News: "I think it reflects very poorly on our country. I think it’s sad all the way around tonight."

Friday's events occurred four days before key votes in five states: Illinois, Missouri, Ohio, Florida and North Carolina. If Trump's momentum continues through those votes on Tuesday, he will be in the best position of any of the four candidates in the race to capture the GOP nomination to run for president.

Trump has won a majority of contests in the state-by-state primary process by which parties choose their presidential candidates, but continues to face sharp opposition from much of the party's political establishment.

Rubio's campaign told voters to support another candidate, John Kasich, in order to stop Trump from winning Kasich's home state of Ohio next week.

"If you’re a Republican primary voter in Ohio and you want to beat Donald Trump, your best chance in Ohio is John Kasich, because John Kasich is the sitting governor and he’s closest to Donald Trump in some of the polls there," Rubio's campaign manager Alex Conant said on CNN.

Conant likewise called for those who want to defeat Trump to support Rubio in his home state of Florida.

The remarks are part of a strategy to divide enough delegates to prevent Trump from winning the Republican nomination outright at the party convention in July.

The Trump campaign received a boost at the start of the day however when a former campaign rival, Ben Carson, endorsed him.

"I want the voice of the people to be heard, I want the political process to play out as it should," Carson said at a news conference in Florida.

Carson said there were "two Donald Trumps" - the outspoken public persona and a "very cerebral" man.

"He's actually a very intelligent man who cares deeply about America," Carson said.

Trump said Carson would have a "big part" in Trump's campaign, but said no deals had been made to give him a prominent position in a potential Trump White House.

Trump has also won the endorsement of one other former Republican contender, New Jersey Governor Chris Christie.

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