The FBI has warned of the potential for armed clashes linked to Tuesday's United States election in Portland, as the northwestern city that has become symbolic of the country's stark divisions braces for unrest.

The liberal enclave in the state of Oregon is still reeling from a summer that saw mass anti-racism rallies inflamed by the arrival of federal officers and right-wing militias, including the so-called Proud Boys.

Tuesday's fiercely polarised vote — which could see President Donald Trump reelected, or defeated by his Democratic rival Joe Biden — has spurred fears of more deadly street violence.

Downtown businesses were boarding up windows once again as protests are planned for either a Trump or a Biden win — or a state of limbo, with delays in the vote-counting expected nationwide due to a surge in mail-in voting during the pandemic.

“The thing that is the most concerning to me is the potential for armed clashes between opposing groups,” FBI Portland Special Agent Renn Cannon told AFP.

“That could escalate into a dangerous situation where — if tempers are heated — you could end up with an unfortunate or tragic act of violence,” he added, pointing to a deadly shooting of a far-right supporter in the city in August.

The 250-strong Portland office has devoted additional resources to election crimes including voter suppression as well as fraud and foreign cyber threats, said Cannon.

Meanwhile Governor Kate Brown on Monday issued an executive order handing Portland policing to state forces — effectively overruling the city's ban on tear gas — and putting the National Guard on standby.

“This is an election like no other in our lifetime,” she warned.

'Wild card situation'

Brown's warnings about white supremacists have drawn scorn from local conservatives including talk-radio host Lars Larson, who Monday accused her of “deafening silence” about “Antifa and Black Lives Matter violence” over five months of protests.

But while Oregon is a safe Democratic state, Portland's Republican hinterland has made it a focus for protests from all ideologies, with further demonstrators flying in from across the country this summer.

FBI agents are being “extra attentive” to any threats that could “reduce the ability for people to exercise their first amendment rights or exercise the right to vote,” said Cannon.

Officials' fears of renewed violence were echoed by voters on Monday, including restaurant cook Leigh Smith.

“I've seen everybody's boarding up already and I'm like 'oh geez,'” said the 35-year-old, after mailing her ballot near the downtown courthouse that became an epicenter of earlier demonstrations.

“It's really a wild card situation. It could be really chill [...] it could become chaotic.” One cause for optimism is that Oregon votes entirely by mail, making lengthy voting lines that could be targeted unlikely on Tuesday, said Cannon.

Of greater concern are multiple protests planned in Oregon for the aftermath of a vote which may not yield a result for days or even weeks, he added.

“Whether or not those will have an armed component or not, I don't know,” said Cannon, with no specific threats currently identified.

'Prepared' for violence

One group organising a rally — the left-wing Democratic Socialists of America's (DSA) Portland branch — told AFP it was “prepared for right-wing street violence to express frustration about their candidate not winning” if Joe Biden triumphs.

“It's our duty to show up and counter them,” said co-chair Olivia Katbi Smith, adding that if protesters fail to mobilise in numbers, militias “will actually drive around and assault people.” DSA Portland does not advocate for armed response to right-wing extremists, she added.

If Trump tries to claim an illegitimate victory, Katbi Smith hopes liberal groups will bring out protest numbers approaching the tens of thousands who attended Portland's 2017 women's rights march.

“We're going to go forward with specific demands about democracy,” said Katbi Smith, including Trump's removal or a new vote.

She added: “There will be right-wing mobilisations against us after the election.“