PORTLAND, OREGON (REUTERS) - State police and law enforcement from neighbouring suburbs were sent to Portland, Oregon, on Monday (Aug 31) as tensions mounted following a fatal weekend shooting in the midst of clashes between supporters of President Donald Trump and counter-protesters.

Portland Mayor Ted Wheeler on Sunday warned of an escalation of protest-related violence after seeing a flurry of social media posts vowing revenge for Saturday's shooting victim, identified by the right-wing Patriot Prayer militia group as one of its supporters.

Governor Kate Brown, a Democrat, announced a plan Sunday she said would allow for peaceful protest while cracking down on lawlessness that has often accompanied demonstrations since the May 25 killing of Mr George Floyd, the Black man who died under the knee of a white police officer in Minneapolis.

"Every Oregonian has the right to freely express their views without fear of deadly violence. I will not allow Patriot Prayer and armed white supremacists to bring more bloodshed to our streets," Ms Brown said in a statement.

In addition to temporarily sending in state police troopers and officers from neighbouring jurisdictions, the FBI and federal prosecutors will add resources to prosecute criminal offences stemming from protests, Ms Brown said.

No mention was made of activating the National Guard. Last week, 1,000 guard troops were sent to Kenosha, Wisconsin, after a wave of arson fires and the deadly shooting of two people protesting in support of a Black man shot in the back and paralysed by a white officer.

Portland's demonstrators, demanding reforms of police practices they view as racist and abusive, have frequently clashed with law enforcement and on occasion with counter-protesters associated with right-wing militia groups.

Hundreds have been arrested, including 29 accused of taking part in an unlawful assembly Sunday night. Two of them were found with loaded handguns, while others were carrying knives and rocks, police said.

The Trump administration in July deployed federal forces to Portland to protect a US courthouse that has been the focus of violent protests and other federal property, drawing criticism that their presence only heightened the unrest. The federal agents were later withdrawn.

STREET CLASHES AND DEADLY SHOOTING

Tensions flared again Saturday night between anti-racism protesters and a large caravan of demonstrators driving through the city's downtown district in pickup trucks waving pro-Trump flags.

Video on social media showed individuals in the trucks firing paint-balls and spraying chemical irritants at opposing protesters as they rode, while those on the street hurled objects at the trucks and tried to block them.

Investigators have yet to say whether the fatal shooting had anything to do with those confrontations. Police identified the victim on Monday as Aaron J. Danielson, 39, of Portland, who died from a gunshot wound to the chest.

Patriot Prayer leader Joey Gibson, speaking to local radio, described Mr Danielson as a "very gentle, very courageous" person whom he had never seen become violent or even angry.

"It doesn't matter what you believe in politically ... if you knew him as a person, you would never hurt him," Mr Gibson said.

Another associate, identified in a YouTube interview as Mr Chandler Pappas, said he was with Mr Danielson when they were confronted by assailants on the street.

"They hunted us down. They recognised our Patriot Prayer hats," Mr Pappas said. Afterward, he recounted, "The shooter took off running."

Mr Pappas said there was no precipitating incident. "The two of us alone, away from the rest of the Trump rally with all the trucks and all the other noise going on - I think they isolated us and they thought they could get away with it," Mr Pappas said, adding that neither he nor Mr Danielson were armed.

Mr Luke Carrillo, a longtime friend and business partner, told reporters Mr Danielson was a "proud American" but "was not a radical, not a racist, not a fascist." And he called for all parties in Portland to "renounce any further acts of violence."

Mr Trump has seized on civil disturbances in cities such as Portland and Kenosha, blaming state and local Democratic leaders, as he amps up his calls for law and order ahead of the Nov 3 election.

"Portland is a mess, and it has been for many years. If this joke of a mayor doesn't clean it up, we will go in and do it for them!" Mr Trump wrote on Twitter on Monday.

Democrats have countered that Mr Trump is stoking violence and racial strife with divisive rhetoric that has emboldened right-wing activists.

"This president long ago forfeited any moral leadership in this country," Democratic presidential nominee Joe Biden said during a visit to Pittsburgh on Monday. "He can't stop the violence - because for years he has fomented it."