Police declared the protest in front of the union building a "riot" following the leading to a standoff that lasted into the night.
About 20 people allegedly broke the glass in the door of the police union (Portland Police Association, or PPA) offices and set a fire, while hundreds more assembled peacefully outside the building, OregonLive .
"At 11.35pm, people within the crowd broke the window to the PPA Office, unlawfully entered, and started a fire, committing the crimes of criminal mischief, burglary, and attempted arson," police said in a .
Security camera footage from inside the building, released by the police, shows people throwing five to ten pieces of burning cardboard into a corner of the office through the broken door.
The flames rise without spreading to the rest of the office and then begin to subside before the video ends.
Three separate crowds marched through the streets of Portland on Saturday, with two of the protests remaining peaceful throughout, police said.
Police had deemed the protest that ended up at the PPA headquarters "peaceful" until 10.30pm.
At that time, an unspecified number of protesters "committed crimes when they erected a fence, pushed dumpsters into the street to block traffic, set a dumpster on fire, vandalized the PPA office with spray paint, and destroyed security cameras," the police statement said.
Following the alleged attempted arson an hour later, police drove protesters into nearby Kenton Park, then ordered them to disperse.
In its statement, the police department denied using tear gas that night.
Protesters threw objects including paint-filled balloons at the police and attempted to shine lasers into their eyes, while police used pepper spray and impact munitions on the crowd and released an "irritant" into the air, Oregon Live .
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In its statement, the police department denied using tear gas that night.
The protest dispersed around 2 a.m. following an unspecified number of arrests, police said, adding that three officers had been injured.
The protests in Portland have drawn national attention following the of federal agents from the Department of Homeland Security to police protesters.