A rally in Oregon sponsored by the right-wing group Patriot Prayer turned violent on Saturday as demonstrators clashed with anti-fascist protesters, who showed up in opposition to the event.

The Portland Police Bureau said that four people were taken to the hospital as a result of clashes at the gathering. One person suffered a serious, but non-life-threatening injury, while three others suffered non-serious injuries, the police said.

Four people were arrested in connection with criminal investigations that began before the Saturday protest, police added.

Amid the fighting, police declared that the so-called “Freedom and Courage” rally had turned into a riot and revoked the Patriot Prayer’s permit to hold the event at the Terry Schrunk Plaza, a federal park, in downtown Portland.

More than 100 demonstrators attended the event in support of Patriot Prayer, while more than 100 counterprotesters showed up, local news station KOIN 6 reported. Members of the Proud Boys, designated by the Southern Poverty Law Center as a hate group, attended the rally. (Patriot Prayer is not a designated as a hate group by the center.)

Counterprotesters threw eggs, half-empty water bottles and firecrackers at demonstrators shortly after the march by right-wingers began, The Oregonian reported.

In response, according to the paper, police fired pepper spray-filled paintballs into the crowd. Police also used firecrackers, smoke bombs and flash bangs to deter the violence, KOIN 6 reported.

Police denied using tear gas during the protest but said they did use “aerial distraction devices.”

Officers with the Department of Homeland Security were also at the demonstration.

Videos filmed by journalists showed Patriot Prayer demonstrators and counter-protesters charging at each other and brawling on the streets while explosions erupted around them.

In a series of tweets, freelance reporter Mike Bivins said that counterprotesters were throwing eggs, garbage and  rocks at Patriot Prayer demonstrators, then things “ratcheted up.”

Jason Wilson, a writer for The Guardian who was covering the rally, said in a tweet that the confrontation between the groups “would have to be among the worst protest violence I have seen in Portland” and included “unfair fights.”

Police reported seeing assaults, criminal behavior and “projectiles being thrown” during the rally. Earlier Saturday, police said they made “probable cause” arrests and tweeted photos of weapons they seized during the demonstration.

The “Freedom and Courage” event was organized by Patriot Prayer leader Joey Gibson, who’s hosted past rallies that have also turned violent and attracted white nationalists.

A group that identifies on Facebook as Eugene Antifa ― based in Eugene, Oregon ― organized the counterprotest, which they named “Defend PDX: Patriot Prayer’s Violence Must End.” The group also identified Gibson and the Proud Boys as their main opponents.

“It is very important that antifascists keep up the pressure so that we can stop them once and for all,” a Facebook event page for the counterprotest read.

“While many acolytes have broken away from Gibson at this point, the core that remains, largely Proud Boys, have grown increasingly violent,” the page said. “For this reason we must sustain our campaign, and keep up our efforts on as many fronts as possible until they completely cease their despicable activity.”

In anticipation of the rally, Portland police said there would be “significant law enforcement presence” in the downtown due to previous protests that erupted in violence between the groups.

Clashes between Patriot Prayer and their opponents broke out during a rally on June 3, leading to at least four arrests. The groups also dueled at several rallies held in 2017.

CORRECTION: A previous version of this story misstated the name of the Portland Police Bureau.