Donald Trump has been caught up in an increasingly angry row over violence and the apparent targeting of African-Americans at his rallies after the circulation of a new video showing a protester apparently being punched by a white supporter.

Invited to express concern, the property tycoon declined and instead offered his praise to supporters who “swing back” at those who disrupt his speeches.

It came just hours before Mr Trump was forced to cancel a campaign rally in Chicago, after police expressed concerns over the thousands of protesters who had turned out to speak against him.

Amid clashes outside the University of Illinois at Chicago Pavilion where he was due to speak, the candidate's campaign team issued a statement saying "tonight's rally will be postponed to another date".

Earlier, Mr Trump was pressed on the issue at his Mar-a-Lago club, moments after he unveiled the endorsement of Dr Ben Carson - once a bitter rival for the party nomination.

He responded by recalling another event two weeks ago, after a man causing a ruckus was escorted out by security guards, when he told the crowd that he himself felt like punching the protester in the face.

“There was that particular one where I said bang him,” Mr Trump recounted at the press conference, with the softly spoken Mr Carson, who had just denounced “strife and division” in the Republican race for president, standing behind him. “The guy was very loud and he started swinging at the audience. And you know what? The audience swung back and I thought it was very, very appropriate.”

Before the press conference, Mr Trump’s campaign manager, Corey Lewandowski, told The Independent that steps had been taken to increase the police and security inside Trump rallies. “If there is anybody out of line, those people are held accountable,” he said.

But he appeared to exonerate those supporters who may be on short fuses, putting it down to their anger at how America has been treated by foreign nations. Mr Trump often rails specifically at Mexico and China for allegedly cheating America on trade.

“Mr Trump’s people are very, very passionate,” Mr Lewandowski said. “They are angry because of the way this country has been taken advantage of by so many other countries. That is the frustration level that a lot of people in this country feel, and people express it in different ways.”

The 78-year-old supporter who was captured in the video that emerged on Thursday said of the protester in a television interview: “The next time we see him, we might have to kill him.” The Trump supporter was arrested and charged.

The incident of the black man being punched is not isolated. There have been two recent cases of media reporters being roughed up or manhandled at Trump events, one allegedly involving a campaign official and a female reporter for Breitbart News, a conservative outlet. A few days earlier, secret service agents tackled a Time photographer before the start of a Trump rally.

But the apparently racially tinged incidents will get the most attention. Another video came out showing a young black student also at the North Carolina event being approached by officers, manhandled to the ground, handcuffed and thrown out. The apparent reason: a group of black men near him had been loudly protesting.

“At first I refused to leave because it was all just a product of being black at a Trump rally,” Adedayo Adeniyi said later. “Basically in that environment I’m guilty by association. But common sense told me to just listen to the cops and move along.

“These are presidential rallies. I’m not at a KKK rally. The fact that I experienced hate at a candidate rally tells you everything you need to know about Donald Trump and the people that support him. People will act more hateful and racist in environments that they feel not only encouraged to do it, but accept it as normal.”

Confirming his decision to back Mr Trump, Dr Ben Carson, who dropped his bid for the Republican nomination just over a week ago, said the two of them had themselves “buried the hatchet”. He added there was a “cerebral” side to Mr Trump that people don’t see.

“There are two different Donald Trumps,” he suggested. “The one you see on the stage and the one who is very cerebral who sits there and considers things very carefully… and that’s the Donald Trump you are going to start seeing more and more of.”

His famous hair given a lavender hue by the lighting in the club’s White and Gold Ballroom, Mr Trump also referred to the past bad blood between him and Dr Carson, but insisted it had meant nothing in the long run. “It’s politics,” he said. “It’s tough stuff… but I was really impressed with the way he fought back. He did so well.”