Clashes broke out Friday at an arena in Chicago prior to a rally by US Republican presidential candidate Donald Trump, prompting him to cancel the event only minutes before it was scheduled to begin.

Tens of thousands of people were at the venue, including thousands of protesters inside and outside the arena, according to news reports.

Protesters inside the arena cheered at the announcement that Trump was pulling out. "We stopped Trump," they chanted, according to the Chicago Tribune.

The crowd of protesters outside the venue had grown throughout the afternoon, reportedly causing concern among police.

The billionaire businessman's campaign said he decided to cancel the rally "for the safety of all of the tens of thousands of people that have gathered in and around the arena," after meeting law enforcement officials in Chicago.

The rally will be held on another date, the campaign statement said. It also thanked people for attending and urged them to "please go in peace."

Police cleared the arena and the area outside without incident, news reports said. The Chicago Police Department reported no arrests on its Twitter site.

In an interview afterwards with broadcaster 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 said there were "minor skirmishes," which reflected divisions in the country over the lack of wage increases, job instability and worry over the economy.

But the Republican front-runner used a different tone earlier in the day when protesters interrupted his rally in St Louis, Missouri. He called them "troublemakers" and ordered them to "go home and get a job," according to the Chicago Tribune.

Even before the events on Friday, Trump had been criticized for telling security to "get them out of here" when protesters interrupted his speeches.

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.

US Senator Marco 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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