Police arrested dozens of people in Seattle, Minneapolis and Portland, Oregon overnight during protests demanding a tally of all votes in the US election, and smaller groups backing President Donald Trump returned to tabulation sites in closely contested states to insist counting be halted.

In Seattle, seven people were arrested. One person arrested for allegedly damaging property was taken to a hospital after "experiencing a medical episode," police said in a statement early Thursday.

The protests came as the president insisted, without evidence, that there were major problems with voting and counting of ballots. Republicans filed suit in multiple states, preparing to contest election results.

In Minneapolis, police arrested more than 600 demonstrators who marched onto an interstate in Minneapolis Wednesday night protesting Trump's threats to challenge the election results, as well as a variety of social injustices.

No force or chemicals were used to make the arrests for walking on a freeway and being a public nuisance, the Minnesota Department of Public Safety said Thursday.

In Portland, protesters smashed windows at businesses, hurled objects including fireworks at officers. Police made at least 10 arrests, according to a statement from the Multnomah County Sheriff's Office.

Officers seized multiple firearms, ammunition, a knife, fireworks, body armor and gas masks from people who were arrested, a sheriff's office statement said.

One of the people who was arrested had a rifle with a magazine of ammunition, fireworks, a knife and was wearing a ballistics vest, the sheriff's office said.

Oregon Gov. Kate Brown called out the National Guard to manage the unrest in Portland, which has been a scene of regular protests for months.

"It's important to trust the process, and the system that has ensured free and fair elections in this country through the decades, even in times of great crisis," Brown said in a statement.

Portland protester Richard March said he came despite a heart a heart condition that makes him vulnerable to COVID-19.

"To cast doubt on this election has terrible consequences for our democracy," he said. "I think we are a very polarized society now — and I'm worried about what's going to come in the next days and weeks and months."

In New York, hundreds of people paraded past boarded-up luxury stores on Manhattan's Fifth Avenue, and in Chicago, demonstrators marched through downtown and along a street across the river from Trump Tower. Protesters also gathered in cities including Los Angeles, Houston, Pittsburgh and San Diego.

The protests came as smaller groups of Trump supporters gathered at vote tabulation sites in Phoenix, Detroit and Philadelphia, decrying counts that showed Democrat Joe Biden leading or gaining ground.

In Phoenix, at least two dozen Trump supporters gathered outside city hall Thursday morning, chanting "Protect Our Vote." The group said they planned to return to the tabulation center, where a Wednesday night rally decried a declaration by Fox News that Biden was the winner in Arizona.

"We're not going to let this election be stolen. Period," said Rep. Paul Gosar, an Arizona Republican and staunch Trump supporter, told the crowd Wednesday.

Observers from both major political parties were inside the election center as ballots were processed and counted, and the procedure was live-streamed online at all times.

Several sheriff's deputies blocked the entrance to the building. The vote-counting went on into the night, Maricopa County Elections Department spokeswoman Megan Gilbertson said.

"Everyone should want all the votes to be counted, whether they were mailed or cast in person," said the statement issued by two top county officials — one a Democrat and the other a Republican. "An accurate vote takes time. ... This is evidence of democracy, not fraud."

In Detroit, a few dozen Trump supporters gathered outside the city's convention center Thursday morning, as election workers counted absentee ballots inside. They held signs that said "stop the steal" and "stop the cheat."

A small group of counter protesters gathered on the other side of the street, and the two sides shouted at each other. Trump's supporters occasionally mocked those on the other side over a loud speaker.

Michigan Attorney General Dana Nessel, a Democrat, insisted Wednesday that both parties and the public had been given access to the tallying, "using a robust system of checks and balances to ensure that all ballots are counted fairly and accurately."