Updated 7:42 p.m. Aug. 17.

Police in Portland, Oregon, arrested at least four people Saturday, established concrete barriers, closed streets and bridges, and seized a multitude of weapons to try to preempt violence between right-wing groups and anti-fascist counterprotesters.

Metal poles, bear spray, shields and other weapons were taken from protesters by the authorities Saturday as hundreds of far-right protesters and counterdemonstrators crowded the downtown area.

Portland Mayor Ted Wheeler said people who espoused hate or engaged in violence were ``not welcome,'' calling the showdown ``potentially dangerous and volatile.'' By midafternoon local time, though, most of the right-wing groups had left the area, using 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. Hundreds of people remained on downtown streets.

Flag-waving members of the Proud Boys and Three Percenters militia groups had gathered late in the morning, some also wearing body armor and helmets. Police said they seized the weapons as the protesters assembled along the Willamette River that 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 reportedly drew people from across the country. Portland police said all the city's 1,000 officers would be on duty for the gathering, which had been publicized on social media and elsewhere for weeks.

U.S. President Donald Trump indicated Saturday morning that he could take action on antifa. The president said in a tweet, "Major consideration is being given to naming ANTIFA an 'ORGANIZATION OF TERROR.' Portland is being watched very closely. Hopefully the Mayor will be able to properly do his job!"

Portland Police Arrest Leader of Right-Wing Group Ahead of High-Profile Rallies The city braces for trouble ahead of opposing demonstrations by far-right groups and anti-fascist counter protesters

On Friday, police arrested Joey Gibson, the leader of the right-wing group Patriot Prayer, prompting him to urge his followers on social media to "show up ten-fold, one hundred-fold" for Saturday's protests.

Gibson, who was not involved in organizing this weekend's event but has planned similar rallies in the past, surrendered Friday on an outstanding warrant for a fight that broke out in May between his right-wing supporters and left-wing adversaries.

The rally was organized by a member of the Proud Boys, which has been designated as a hate group by the Southern Poverty Law Center. Joining them were the American Guard, Three Percenters, Oathkeepers and Daily Stormers.

According to the Southern Poverty Law Center, the American Guard is a "white nationalist group"; Three Percenters and Oathkeepers are "extremist" anti-government militias; and the Daily Stormers are "neo-Nazis."

Countering the right-wingers was Portland's Rose City Antifa, a local anti-fascist group that called on its members to take to the streets in an opposing rally.

Antifa has grown more visible recently. Experts say the groups are not centrally organized, and their members may focus on a number of causes, from politics to race relations to gay rights. But the principle that binds them - along with an unofficial uniform of black clothing and face masks - is the willingness to use violence to fight white supremacists, which has opened them to criticism from both left and right.

At a June rally in Portland, masked antifa members beat up a conservative blogger named Andy Ngo. Video of the 30-second attack grabbed national attention.

The city's leadership and residents were on edge ahead of the rallies. Many summer staples like music festivals and recreational events were cancelled. A 5K race changed its course to avoid possible violence, and most businesses in the area planned to close.