Secret Service agents have had to rush on stage to protect Donald Trump during a disturbance at a rally, a day after rowdy protests shut down his event in Chicago.
Mr Trump, who leads the race for the Republican nomination, briefly ducked at the podium and four agents scrambled to surround him after a man charged the stage at Dayton airport in Ohio.
The man was hustled away and has been arrested.
At one rally, Mr Trump had a message for protesters.
"By the way, what do they do, do they arrest these people? And some of them are very violent, but I'm going ask that you arrest them, I'm going to file whatever charges you want, who the hell knows.
"You know what, if they want to do this - I say to the rest of the room - because we're going to go strongly for your arrest and i'm going to do this from now on. Let's ruin the their, the rest - they're going to ruin the rest of their lives. If they want to do this let them have a big arrest mark, they got arrested.
Mr Trump had to cancel his Chicago rally the day before, after fighting broke out between his supporters and protesters.
Those clashes began more than an hour before the event was due to start, and continued after it was cancelled.
Violent scuffles were sparked by Trump supporters attempting to wrestle flags from protesters.
One protester had to be physically removed from the stage. The clashes continued outside the venue.
Donald Trump's Republican rivals condemned the Chicago protests, but said his inflammatory speeches were partly to blame.
Mr Trump said the protesters were "thugs" and put some of the blame for the chaotic scenes on supporters of Democratic Party contender Senator Bernie Sanders.
"By the way, some represented Bernie, our communist friend," he said.
"Now really Bernie should tell his people - there weren't too many Hillary people there, you know why - I mean in all fairness to Bernie the reason there weren't Hillary people [is] they have no fervour."
Mr Sanders hit back, calling Mr Trump "a pathological liar".
"Obviously, while I appreciate that we had supporters at Trump's rally in Chicago, our campaign did not organise the protests," he said.
Obama calls on contenders not to raise tensions
US President Barack Obama warned White House contenders to avoid raising tensions, a day after a rally by Donald Trump was called off amid clashes.
Mr Obama said candidates should not resort to "insults" and "certainly not violence against other Americans".
Mr Obama, who would be standing down after November's presidential election, was speaking at a Democratic Party fundraiser in Dallas on Saturday.
"What the folks who are running for office should be focused on is how we can make it even better - not insults and schoolyard taunts and manufacturing facts, not divisiveness along the lines of race and faith," he said.
Mr Trump campaigned in Ohio on Saturday. It was one of several states, including delegate-rich Florida, as well as Illinois, Missouri, North Carolina and the Northern Mariana Island's Republican caucus, set to hold primaries on Tuesday.
Mr Trump had a strong anti-immigrant stance, promising to build a "great wall" at the border with Mexico.
Commenting on relations between Muslims and America earlier this week, he said "Islam hates us".
Speaking to Fox News after Friday's events in Chicago, Mr Trump denied fostering division.
"I represent a large group of people that have a lot of anger," he said. "There is tremendous anger out there on both sides."
Mr Trump's rivals for the Republican nomination, Marco Rubio and Ted Cruz, both called the incident that cancelled the Chicago rally "sad".
Mr Cruz accused Mr Trump of creating "an environment that only encourages this sort of nasty discourse".
Mr Rubio and another Republican challenger, John Kasich, suggested they might not rally behind Mr Trump if he won the nomination.
Mr Rubio said it was "getting harder every day" to keep his promise to unite behind the eventual Republican nominee.
Mr Kasich said Mr Trump's rhetoric made it "very difficult" to support him.
On Saturday, party contests would be held in Wyoming, Washington DC, the Northern Mariana Islands and the nearby island territory of Guam.
In the Democratic race, Vermont senator Bernie Sanders was continuing his challenge against frontrunner Hillary Clinton.