Donald Trump defended Sunday his campaign manager, who appeared to grab a protester at a rally for the Republican presidential frontrunner that was again marred by demonstrations and sporadic violence.

It is just the latest controversial turn in the real-estate mogul's unconventional run for the White House, but it appears to have done little harm so far to his presidential ambitions and he is in pole position for the Republican nomination for November's election.

On Saturday afternoon anti-Trump protesters blocked a road near Phoenix and in the evening there was more unrest at a Trump rally in Tucson, also in Arizona, that was frequently disrupted by demonstrators.

Among the incidents, cameras caught a member of the crowd repeatedly kicking and punching a protester and video also showed Trump's campaign manager Corey Lewandowski caught up in a melee.

Lewandowski appeared to grab one man by the scruff of the neck, with a member of Trump's security detail also involved in the fracas.

"No, he didn't touch him, that was someone else pulling him (the man) back," Trump, a reality television star who has never held elected office, told the ABC News program "This Week."

"I give him credit for having spirit. He wanted them to take down those horrible profanity-laced signs," Trump said of his campaign manager.

Trump's run for the White House has from the start attracted boisterous crowds and counter demonstrators, but the rallies have taken a darker turn in the last couple of weeks, beginning when an elderly white Trump supporter sucker-punched a black demonstrator in North Carolina.

A Chicago Trump rally was then cancelled after demonstrators scuffled with his supporters and police struggled to maintain order, with hundreds of protesters showing up.

Trump has came under fire from his Republican and Democratic rivals, who blame his incendiary rhetoric on everything from immigrants to Muslims for the violence.

"We don't condone violence and I say it," Trump told ABC, asked if would condemn the incident that saw a man viciously punching and kicking to the floor a demonstrator wearing an American flag shirt. The attacker was arrested and charged with assault with injury, NBC News said.

Video also showed someone -- whom Trump called "a disgusting guy" -- wearing a white mask similar to the Ku Klux Klan and giving what seemed to be a Nazi salute.

Trump, after adding to his growing pile of primary wins but losing in the key state of Ohio, on Wednesday warned of riots if he was denied the Republican presidential nomination, in one of the most divisive US campaigns in nearly half a century.

Asked if would tell his supporters to riot if he loses out, he said Sunday: "I don't want to see riots. I don't want to see problems."