The man shot and killed in Portland on Saturday was a supporter of the right-wing group Patriot Prayer whose supporters have repeatedly clashed with left-wing protesters in Oregon’s largest city, according to the organisation’s founder.

Joey Gibson, head of the group based in Washington state, said he was also in Portland on Saturday night when supporters of US President Donald Trump clashed with Black Lives Matter protesters.

Mr Gibson said he arrived at the scene of the shooting shortly after it happened and was briefly corralled in a nearby petrol station by angry protesters.

“I can’t say much right now. All I can do is verify that he was a good friend and a supporter of Patriot Prayer,” Mr Gibson said about the victim.

Police are seeking videos, photos or eyewitness accounts of the incident, which happened about 15 minutes after a caravan of about 600 vehicles that formed part of a pro-Trump rally left the city centre.

It is not clear whether the shooting was related to the clashes between Trump supporters and counter-protesters in Portland. The city has become a flashpoint in the national Black Lives Matter protests since George Floyd was killed in May, and an increasing centrepiece in Mr Trump’s law-and-order re-election campaign theme.

A Trump supporter and a counter protester scuffle. Photo: Getty

Police have not released any information about the case but said they were aware of videos on social media that showed the shooting.

“It is still early in this investigation, and I ask everyone to give the detectives time to do their important work before drawing conclusions about what took place,” Police Chief Chuck Lovell said.

“If anyone can provide information about this case, I ask them to please reach out to our detectives.

“This violence is completely unacceptable and we are working diligently to find and apprehend the individual or individuals responsible.”

Portland has been the site of nightly protests for more than three months since the police killing of Mr Floyd in Minneapolis.

Protesters at a demonstration to commemorate Martin Luther King’s 1963 ‘I Have a Dream’ speech.

Many of them end in vandalism and violence, and hundreds of demonstrators have been arrested by local and federal law enforcement since late May.

In the two hours following Saturday’s shooting, protesters gathered in the city centre and there was sporadic fighting and vandalism. Police said 10 people were arrested.

The caravan had arrived in the city centre just as a planned protest was getting underway.

The chaotic scene came two days after Mr Trump invoked Portland as a liberal city overrun with violence in a speech at the Republican National Convention.

The caravan marked the third Saturday in a row that Trump supporters have rallied in the city.

Patriot Prayer has a history in Portland and has repeatedly rallied supporters for pro-Trump rallies beginning in 2017.

The group’s supporters periodically cross into Oregon for rallies and marches.

Counter-protesters have squared off with Patriot Prayer and other right-wing groups such as the Proud Boys and the Three Percenters multiple times in the city since Mr Trump’s election.

Mr Trump issued a flurry of tweets and retweets on Sunday, including several that blamed Portland mayor Ted Wheeler for the death and one in which the President appeared to be encouraging his supporters to move into Portland.

“Great Patriots!” Mr Trump wrote as he shared video of his supporters driving into Portland to confront the protesters.

Acting homeland security secretary Chad Wolf blamed local officials for failing “to protect their communities”.

-AAP