"Preliminary information indicates that a suspect was in a vehicle on the bridge and there was a confrontation with someone in the protest," Portland police said in a statement. "The suspect got out of the vehicle and fired multiple shots injuring the victim." The victim is expected to survive, police said.
The arrests were made after an off-duty officer spotted a suspect vehicle and detained four people who are believed to be "criminal gang associates," police said. A firearm was found in the vehicle, police said.
Earlier, police reported that "burning projectiles" were being thrown at officers during one of three demonstrations that occurred simultaneously in the city.
The gunshots came on the fourth night of protests in Portland. Protesters hit the streets again Saturday on a fifth night of demonstrations since the election. There were three arrests, police said.
On Thursday, a similar rally boiled over into what police described as a "riot" after some demonstrators armed with bats smashed stores and cars, and others lit fires.