"@PortlandPolice has declared the gathering near the Multnomah Building a riot after individuals vandalized, repeatedly smashed first floor windows with rocks and threw burning material into an office", the office wrote on Twitter.
Local media reported that protesters gathered in Colonel Summers Park on Tuesday evening, marking the 83rd consecutive night of rallies in Portland, to march to the Multnomah County Building.
The march was peaceful for about an hour, but after protesters arrived at the destination, they started to lit fires in dumpsters and brake windows.
Protests against police brutality and racism have erupted in Portland and other US cities after the death of African American man George Floyd in police custody on 25 May. The Portland protests have since evolved into a nightly battle against the federal forces that President Donald Trump deployed to the city despite the objections of state and local officials.