WASHINGTON (AFP) - Presumptive United States Republican presidential nominee Donald Trump on Friday (June 3) denounced as "thugs" the violent protesters outside one of his rallies in California the previous evening.

"Rally last night in San Jose was great. Tremendous love and enthusiasm in the hall. Big crowd. Outside, small group of thugs burned (American) flag!" Trump said on Twitter.

In another posting, he said: "Great evening in San Jose other than the thugs. My supporters are far tougher if they want to be, but fortunately they are not hostile."

Violent clashes between pro- and anti-Trump camps have erupted outside a number of the candidate's rallies in recent weeks.

He has angered many in the US with his rhetoric against Hispanics, women, Muslims and others.

President Barack Obama also weighed in, criticising the political violence in San Jose.

"That's not what our democracy is about. That's not what you do. There's no room for violence. There's no place for shouting," he said at a fundraising dinner in Miami.

"There's no room for a politics that fails to at least listen to the other side - even if you vehemently disagree. Because I believe if you've got the better argument, then you don't need to do that. Just go out there and organize and persuade."

Late Thursday, Trump protesters harassed and scuffled with his supporters outside the rally in the city of San Jose.

One protester was seen burning an American flag.

Hundreds of demonstrators insulted Trump supporters as they tried to leave the event. One supporter was hit with an egg.

A dozen or more people were punched, the Los Angeles Times reported.

Crowds had earlier chanted "No hate in our state" and carried signs that read "Dump Trump" as they marched near the convention center.

California is the most populous state in the country, and one where Hispanics outnumber whites.

It will hold its primary on Tuesday, although Trump is a shoo-in for the Republican contest since all of his opponents have dropped out.