DAYTON, OHIO: Secret Service officers rushed on stage to protect US Republican presidential front-runner Donald Trump during a disturbance at a rally on Saturday, a day after rowdy protests shut down his event in Chicago.

Trump briefly ducked at the podium and four Secret Service members scrambled to surround him after a man charged the stage at Dayton International Airport in Ohio.

Officers then grabbed the man, dressed in a black tee shirt and jeans, before he was able to reach the stage and hauled him away.

"I was ready. I don't know if I would have done well but I would have been out there fighting, folks," Trump told a rally later in the day. He said the man "was looking to do harm."

The incident further increased tension after Trump's Chicago rally was scrapped amid chaotic scenes on Friday.

Trump's Republican rivals hurled scorn at the New York billionaire, saying he helped create the nervous atmosphere that is now sweeping the race for the White House with his fiery rhetoric.

Trump blamed supporters of Democratic candidate Bernie Sanders for the incidents in Chicago, where scuffles broke out between protesters and backers of the real estate magnate. He called the US senator from Vermont "our communist friend."

The scenes in Chicago followed a series of recent incidents of violence at Trump rallies, in which protesters and journalists have been punched, tackled and hustled out of venues, raising concerns about degrading security leading into the Nov. 8 election.

"All of a sudden a planned attack just came out of nowhere," Trump said in Dayton, describing the events in Chicago. He called the protest leaders there "professional people".

Sanders, a US Senator from Vermont, hit 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."

President Barack Obama told a fundraising event in Dallas that political leaders "should be trying to bring us together and not turning us against one another."

Democratic front-runner Hillary Clinton drew criticism for releasing an initial statement that did not mention Trump by name and tied violent campaign events to a shooting in a South Carolina black church last year that left 9 people dead.

While campaigning in St. Louis, Missouri, on Saturday, Clinton criticised Trump directly for "ugly, divisive rhetoric" that encourages aggression and violence.

CRUCIAL PRIMARIES

The months-long Republican race may be coming to a head at nominating contests on Tuesday where Donald Trump is seeking victories that might give him an almost insurmountable lead for the nomination.

Primaries in Florida and Ohio will be particularly important since they are winner-take-all states, where all Republican delegates are given to the winner of the popular vote instead of being awarded proportionally.

It will be a make-or-break day for Republican candidates John Kasich, the governor of Ohio, and US Senator Marco Rubio of Florida, who both must win their home states to forge a credible path forward.

Trump has drawn fervent support as well as criticism for his calls to build a wall along the US-Mexico border and to impose a temporary ban on Muslims entering the country.

His rallies often attract small groups of protesters, but Friday's was the first at which there may have been as many protesters as supporters.

At an event in Kansas City on Saturday, Donald Trump urged police to arrest people who disrupt his events.

"They'll have to explain to Mom and Dad why they have a police record and why they can't get a job. And you know what? I'm going to start pressing charges against all these people and then we won't have a problem," he said to cheers.

Outside the rally, police broke up shouting matches between Trump supporters and protesters who shouted, "Shut it down!"

Police on horseback and riot gear briefly moved into a crowd of protesters and officers used what appeared to be pepper spray against demonstrators for a few seconds.

Rubio bemoaned the state of the presidential race during an event in Pinellas County in Florida, saying it had "become reality television."

"Last night in Chicago, we saw images that make America look like a Third World country," Rubio said, reminding supporters the stakes on Tuesday are high.

Kasich told journalists before a campaign event in Cincinnati, Ohio, that Trump had created a "toxic environment."

Republican candidate US Senator Ted Cruz of Texas called the Chicago incidents "sad."

Vanderbilt University political scientist John Geer said that the tension on display at Trump's events are a proxy for what is going on in the electorate writ large.

"People have strong reactions to Donald Trump," Geer said. "They are playing out in the voting booth and they are also playing out at these events."

Geer said the Chicago cancellation would likely embolden Trump's supporters - an idea floated by Donald Trump in several television interviews.

Clinton picked up four delegates in the Northern Mariana Islands' Democratic primary on Saturday, to Sanders' two.

On the Republican side, Cruz won around two-thirds of the votes in Wyoming's Republican nominating contest but because of the state's unusual rules it is not clear how many Wyoming delegates will go his way at the Republican Convention in July.

Washington DC also held a Republican caucus on Saturday, though results were not yet in.