POLICE in Portland, Oregon, have arrested at least three people during demonstrations by right-wing groups and counterprotesters.

Authorities didn't immediately release details of the arrests. Hundreds of flag-waving members of the Proud Boys and Three Percenters militia group, as well as black-clad anti-fascist counterprotesters, gathered in downtown Portland on Saturday.

They were met by a huge police presence of more than two dozen local, state and federal agencies. Police used concrete barriers and closed bridges to keep the rival groups separated.

Police say they've seized weapons such as metal and wooden poles, bear spray and shields from multiple groups at the rally and counterprotest in the city.

Meanwhile, the leader of a right-wing group was busted by police in Portland, Oregon, before a rally expected to draw people from around the country.

Patriot Prayer's Joey Gibson to urge his followers to "show up one hundred-fold" in response to his arrest, the Associated Press reported.

The arrests of Gibson and five other far-right activists appear to be a signal from cops to organisers to keep their event peaceful, the New York Post said. Anti-fascist groups have promised to confront the right-wingers at their rally in Oregon's largest city. Similar events in the past resulted in widespread unrest on Portland's streets.

An organiser told the Portland Oregonian he expects up to 1000 people to take part in the demonstration. Thousands of counter-protesters are also expected.

CNN reports that Portland authorities have been warning residents to stay home as far-right groups planned a rally aimed at putting an "end to domestic terrorism," with a particular focus on Antifa extremist groups.

There were fears the rally could prompt violent clashes between both a smattering of right-wing extremists and Antifa counterdemonstrators, like those the city has seen in recent years.

The Oath Keepers, an anti-government group associated with the militia movement, said in a statement on its website it would not attend the event as a group but would continue to oppose Antifa.