PORTLAND, Ore. (AP) — Police arrested at least 13 people and seized metal poles, bear spray and other weapons Saturday as hundreds of far-right protesters and anti-fascist counter-demonstrators swarmed downtown Portland.

Authorities closed bridges and streets to try to keep the rival groups apart. The city's mayor said the situation was "potentially dangerous and volatile," and President Donald Trump tweeted "Portland is being watched very closely."

As of early afternoon, most of the right-wing groups had left the area via a downtown bridge. 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.

But hundreds of people remained downtown and on nearby streets, and there were skirmishes throughout the day. Police declared a gathering of mostly left-wing protesters near Pioneer Courthouse Square a "civil disturbance" and told people to leave.

One person was injured and transported via ambulance, and three other people were evaluated by medics, Portland police spokeswoman Lt. Tina Jones said. The injuries were minor, she said.

Jones said at one point there were about 1,200 on the streets, but that number had fallen to about 400 late in the afternoon.

The events began late Saturday morning. Flag-waving members of the Proud Boys, Three Percenters militia group and others gathered downtown, 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 to the event, Portland Mayor Ted Wheeler said people who espoused hate or engaged in violence were "not welcome."

In a Saturday morning tweet, Trump wrote: "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.

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.

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.

Police continue to investigate several incidents from clashes on May 1 and June 29 and are politically neutral, Jones said.

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.

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