PORTLAND, Ore. — Police seized metal poles, bear spray and other weapons Saturday as hundreds of far-right protesters and anti-fascist counter-demonstrators swarmed downtown Portland, Oregon, as police set up concrete barriers and closed streets and bridges in an effort to contain and separate the rival groups.

The city's mayor said the situation was "potentially dangerous and volatile" but as of early afternoon most of the right-wing groups had left the area via a downtown bridge and police used officers on bikes and in riot gear to keep black clad, helmet and mask-wearing anti-fascist protesters — known as antifa — from following them.

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Police said they arrested at least three people. Hundreds of people remained on downtown streets.

Flag-waving members of the Proud Boys and Three Percenters militia group had gathered late in the morning, some also wearing body armor and helmets. Police said they had seized the weapons, including shields, from multiple groups as they assembled along the Willamette River, which runs through the city.

More than two dozen local, state and federal law enforcement agencies, including the FBI, were in the city for the right-wing rally that was expected to draw people from across the country. Portland Police said all of the city's 1,000 officers would be on duty for the gathering that was hyped on social media and elsewhere for weeks.

In the days leading up the event Portland Mayor Ted Wheeler said people who espoused hate or engaged in violence were "not welcome."

In a Saturday morning tweet, President Donald Trump wrote "Portland is being watched very closely" and that "Hopefully the Mayor will be able to properly do his job."

He also wrote that "major consideration is being given to naming ANTIFA an 'ORGANIZATION of TERROR.'"

But it wasn't immediately clear what he meant by that as there's no mechanism for the United States government to declare a domestic organization a terror group. The State Department maintains a list of designated foreign terrorist organizations, such as al Qaida, but there's no comparable designation or list for American groups.

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RELATED: What to know about Saturday’s protests in Portland

Wheeler responded to the president's tweet in an interview with CNN, saying, "frankly, it's not helpful." Wheeler added: "This is a potentially dangerous and volatile situation, and adding to that noise doesn't do anything to support or help the efforts that are going on here in Portland."

Not all who gathered Saturday were with right-wing groups or antifa. Also on hand were people dressed in colorful outfits and those who attended a nearby prayer service, holding signs that said slogans such as "No Trump, No NRA."

Self-described anti-fascists had vowed to confront the rally while leaders from the far right urged their followers to turn out in large numbers to protest the arrests of six members of right-wing groups in the run-up to the event.

RELATED: Portland mayor says police response to Aug. 17 demonstrations will be 'on a scale that this city hasn’t seen in years'

Patriot Prayer's Joey Gibson, who organized similar rallies in 2017 and 2018 that erupted in clashes, surrendered Friday on an arrest warrant for felony rioting. He was at a confrontation that broke out on May 1 outside a bar where antifa members had gathered after a May Day demonstration.

In a video he livestreamed on Facebook, Gibson accused the police of playing politics by arresting him but not the masked demonstrators who beat up conservative blogger Andy Ngo at a June 29 rally that drew national attention.

A video of that attack went viral and led the Proud Boys, who have been designated a hate group by the Southern Poverty Law Center, to organize Saturday's event.

"They want you to not show up in Portland, they want to put fear in your hearts," Gibson said.

Portland Police Bureau spokeswoman Tina Jones declined to comment about specific arrests but said generally police continue to investigate several incidents from clashes on May 1 and June 29 and are politically neutral.

Antifa members often cover their faces with masks, making it harder to identify them.

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In addition to the Proud Boys and Three Percenters, the white nationalist American Guard also said it would have members in Portland.

The Oath Keepers, another far-right militia group, said in a statement they were pulling out of the rally because organizers have not done enough to keep white supremacist groups away.

"It would be best for the patriot/conservative cause if this August 17 rally were simply canceled," the group's founder, Stewart Rhodes, wrote.

Authorities asked residents not to call 911 unless it's a life-threatening emergency and to stay away from the heart of downtown.