Police use chemical irritants and crowd control munitions to disperse protesters in Portland on Saturday. AP

Police arrested dozens of people and used tear gas against hundreds of demonstrators in Portland late Saturday as the western US city marked 100 days since Black Lives Matter protests erupted against racism and police brutality.

Police quickly declared the rally a "riot" after protesters hurled Molotov cocktails at them, and they responded with a barrage of tear gas to disperse the crowd.

At least one person was injured by the fire bombs, police said.

Officers arrested dozens of protesters in an ensuing game of cat-and-mouse through a residential district of east Portland.

"It was Day 100 for Black Lives Matter protests in Portland since George Floyd's murder and they wanted to prevent us from marching," said 20-year-old protestor Jay, her eyes red from the tear gas.

"It's our constitutional right to be here and to express ourselves."

Police said people were "engaging in tumultuous and violent conduct thereby intentionally or recklessly creating a grave risk of causing public alarm."

"This is a riot. Police are giving announcements to disperse. People are throwing Molotov cocktails," Portland Police said on Twitter.

The nightly protests were sparked by the death of George Floyd while in police custody in Minneapolis in May.

Floyd's death triggered similar protests nationwide, but it is in Portland - a city of 650,000 that is more than 70 per cent white - that activists have remained on the streets practically every night, demanding racial justice and police accountability.

President Donald Trump has cast the city as being under siege by "thugs" engaged in "domestic terrorism," though the demonstrations have been peaceful for the most part.

Tensions escalated again last weekend after a man identified as a supporter of a right-wing group was shot and killed.

Earlier on Saturday, hundreds of people gathered in a park near Portland in a memorial for Aaron "Jay" Danielson, a supporter of the far-right group Patriot Prayer.

The 39-year-old was fatally shot after he joined pro-Trump supporters who descended on Portland, sparking confrontations with Black Lives Matter counter-protesters.

The suspect in the shooting, Michael Reinoehl, 48, was killed in neighboring Washington state as police tried to arrest him.

The circumstances of Danielson's death are still under investigation but Reinoehl appeared to acknowledge shooting him in an interview with Vice News.

"We are in the midst of a civil war between good and evil, the good, the good is all the folks you see here," said Dann, 50, who knew Danielson through Patriot Prayer, pointing to the families lined up near an imposing barbecue among camping chairs and numerous American flags displayed for the occasion.

"The evil is the liberals who are letting these cities be destroyed by Antifa, BLM, whoever you want to call them," he said.

Apart from the homage to the victim and the groups of men with assault rifles or pistols, the atmosphere at the park in Vancouver, just north of Portland, was almost partylike.

But Dann was gloomy about the run-up to the presidential election.

"The next two months are going to be terrible. There's going to be more violence. There's going to be more atrocities. There's going to be more name calling," he predicted.

Carrying a skateboard and with a pistol on his belt, Daparu De Crusha, 51, said BLM protestors were trying to destroy the American family.

The former publicist was convinced that Danielson was not killed by chance.

"I believe it was orchestrated, it was a hunting," he said.

"There was going to be a human sacrifice that night and Jay Danielson was their victim," he said.

After citing conspiracy theories about China and Bill Gates, De Crusha said Trump's victory or defeat on Nov.3 would not change the country's problems.

Agence France-Presse