The Portland Police Bureau said the protesters gathered to counter a demonstration outside the Multnomah County Courthouse that was abruptly canceled.
Police did not provide details about the previously planned demonstration, but Oregon Live reported it was rumored to be a Ku Klux Klan white supremacy rally at a downtown park. Counter-protesters, including members of Antifa, still showed up even though police said the original event was scrapped.
"While the rally was being held and was peaceful, Portland Police officers stayed away," the police bureau said. "However, some people in the group began acting in a violent, threatening manner against attendees who were legally capturing photos and videos."
According to police, some people used metal-tipped umbrellas to "jab toward people and chase them down the street." Other objects, such as rocks, concrete, batons, cans and food were thrown at members of the public and officers.
At least two incendiary devices, believed to be flares, were thrown into nearby traffic, police said. The event spanned nearly four hours and required a "large police response" that limited the department's ability to respond to calls for service citywide.
A war memorial in Lownsdale Square Park was marred with spray-painted graffiti.
Messages reading "Punch Cops," "Kill Cops," and "All Cops are Bastards" could be seen on the memorial in photos released by police.
Portland Fire and Rescue medics had to treat three people for exposure to pepper spray wielded by civilians.
"Law enforcement officers did not use force against anyone during this event," the police bureau said.
One vandalism suspect was arrested after a short foot pursuit, while another suspect is at large.
The Multnomah County Sheriff's Office said Willy Cannon, 25, was charged with a misdemeanor for abusing a memorial and a felony for criminal mischief. In addition to Cannon, the sheriff's office said Brandon Farley, 31, was charged with misdemeanor disorderly conduct and Heaven Davis, 19, was charged with misdemeanor criminal mischief.
During the four-hour event, police said a pedestrian was struck by a vehicle and transported to a hospital with a non-life-threatening injury. Police said the driver of the vehicle remained on scene and cooperated with officers.
Police Chief Jami Resch said the actions of the protesters "negatively impacted public safety of the entire city because resources had to be diverted to this event."
"I want to acknowledge the community members who came down to peacefully assemble and exercise their freedom of speech rights," Resch said in a statement. "Unfortunately, a group of people chose to engage in dangerous, illegal behavior. I appreciate the thoughtful, measured response by our officers and law enforcement partners."
Authorities said the investigation continues. Anyone with knowledge of the unknown suspect's identity, or more photos of any crimes that took place during the protest Saturday is urged to email the information to police at email@example.com.