Portland protesters broke into the building of the local Police Association headquarters, setting it on fire late Saturday night, according to police.
The fire was put out a short time later, according to a police statement on Twitter.
Police have declared the gathering a riot and begun working to clear the downtown area.
Portland has been dealing with ongoing anti-police brutality and anti-racism protests and riots since the 25 May death of African American George Floyd while in the custody of white police officers in Minneapolis.
On Friday, Portland Mayor Ted Wheeler called on US President Donald Trump to remove federal troops deployed to the city by the White House to quell ongoing protests, as the agents have reportedly used heavy-handed tactics against protesters, reporters, and legal observers, with some refusing to identify themselves and wearing no identification.