Supporters and opponents of Republican presidential nominee Donald Trump have clashed in the city of San Diego in California.

Police declared a gathering outside the city's convention centre unlawful and made 35 arrests, as stones and water bottles were thrown.

Trump was in the city near the Mexican border to hold a rally ahead of the 7 June California primary.

He has pledged to build a border wall to keep out illegal immigrants.

'FANTASTIC JOB'

The skirmishes flared as the convention centre emptied following Trump's rally, and supporters and opponents met in the streets, jeering and heckling each other

Dozens of police officers in riot gear had been deployed to separate them.

Some protesters scaled a wall of the centre to throw water bottles at police.

Police were deployed to keep the rival camps apart in San Diego. Photo: AFP

After ordering the crowds to disperse, police then moved them away from the city's Gaslamp Quarter.

San Diego's population is about one-third Latino and hundreds of thousands of people cross the border with Mexico legally each day.

One San Diego protester, Martha McPhail, told the local City News Service: "I am opposed to the hateful, bigoted, racist language of Donald Trump and his arrogance and intolerance.

"I'm for all of our people - all races, sexes, genders, military veterans - and he's divisive."

But Trump supporter Riley Hansen defended the controversial businessman.

"My Dad always told me you need a businessman as president. I like his policies," he told CNS.

An anti-Trump protesters is arrested after clashes with police during a rally outside Republican presidential candidate Donald Trump's event in San Diego, California, on May 27, 2016. Photo: AFP

The San Diego Police Department said that 35 arrests had been made and there was no damage to property and no injuries reported.

Trump tweeted to the police after the event: "Fantastic job on handling the thugs who tried to disrupt our very peaceful and well attended rally."

Trump is running unopposed in California after his Republican rivals pulled out and he reached the number of delegates needed to secure the nomination. It has yet to be formalised.

'BULLY'

Earlier on Friday, Trump backed out of an offer to debate with Democratic candidate Bernie Sanders.

Protesters and supporters of Republican presidential candidate Donald Trump argue during a rally outside Trump's event in San Diego, California, on May 27, 2016. Photo: AFP

"As much as I want to debate Bernie Sanders - and it would be an easy payday - I will wait to debate the first place finisher in the Democratic Party, probably Crooked Hillary Clinton, or whoever it may be," the Trump campaign said in a statement.

Sanders told reporters on the campaign trail that he hoped Trump would change his mind.

"Well Trump, what are you afraid of?" he said, calling the Republican nominee a "bully".

Trump said the Democratic nominating process was "rigged" - and that Clinton and Democratic National Committee Chairwoman Deborah Wasserman Schultz would not allow Sanders to win the nomination.

The latest opinion polls suggest Clinton leads Trump by about four percentage points.