Rioters are attempting to break down the door of the Portland Police Association, law enforcement said on Wednesday evening.
"A protest has developed near North Lombard Street and North Campbell Avenue, where participants have tried to break down the door the Portland Police Association," the Portland police tweeted. "We are monitoring the situation."
Police added that those trying to break in were subject to arrest.
"To those participating in the protest, this direction is being given over loudspeaker: stay on the sidewalk, do not try to break into the building or violate any other laws. If you do you're subject to arrest and/or use of force including less lethal weapons."
Police later issued an update that the demonstrators had left the neighborhood. While some vandalism occurred, no arrests were made and no force was used.
The incident came as Portland saw months of unrest. Journalist Andy Ngo, who frequently tracks these types of demonstrations, posted purported footage of recent demonstrations online.
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Left-wing protests in Portland have caused roughly $2.3 million in damage to federal buildings since they broke out over the summer, according to local media.
Fox News' Michael Ruiz contributed to this report.