WASHINGTON: Republican presidential candidate Donald Trump said that "professional agitators" bore much of the blame for violence at his rallies as video showed a protester being beaten and another apparently being grabbed by Trump's campaign manager.

Speaking on ABC's "This Week" on Sunday, Trump defended campaign manager Corey Lewandowski and declined to condemn supporters who have attacked protesters at his increasingly chaotic rallies.

Nor did he back down from his warning that there would be riots in the streets if the Republican Party denied him the nomination for the November election, despite his being the most popular candidate among Republican voters.

Senior figures in the party are openly plotting to prevent Trump from becoming the nominee because they view him as insufficiently conservative, and Trump was due to privately meet with some party leaders in Washington on Monday, the Washington Post reported.

"I don't know what's going to happen, but I will say this, you're going to have a lot of unhappy people," he said on "This Week," predicting anger at the party's national convention in July should someone else end up the nominee. "I don't want to see riots, I don't want to see problems. But you're talking about millions of people."

Scenes of mayhem have become increasingly common at the billionaire New York businessman's rallies, which have been frequently interrupted by protesters, many of them Democrats, who say Trump's controversial remarks on immigrants and Muslims are dangerous. The 69-year-old candidate has sometimes encouraged his supporters using violence on protesters, and on at least one occasion said that he would like to punch a protester himself.

Television footage from an Arizona rally on Saturday showed a man punching and kicking a protester as he was led out of the event. Another video appeared to show Lewandowski grabbing a protester by the back of his shirt.

Trump declined to condemn the violence and said it was often provoked by protesters, who briefly blocked a highway leading to an Arizona rally on Saturday.

"These people are very disruptive people. They're not innocent lambs," he said.

He also defended Lewandowski and said a security official had actually grabbed the protester. Lewandowski also manhandled a reporter last week, according to the Washington Post.

"I give him credit for having spirit," Trump said of Lewandowski.

Republican leaders said Trump needs to more clearly discourage his supporters from engaging in violence.

"All of the candidates for president ought to be discouraging that kind of activity, because people in the audience tend to listen to those who are speaking," Senate Republican Leader Mitch McConnell said on ABC.

About two dozen senior Republican figures will meet with Trump at a law firm near the Capitol on Monday afternoon in what the Trump campaign described as an effort to improve "party unity", the Washington Post reported. The newspaper did not say who would be attending, but McConnell said he would not be in the city that day.