In a statement on Twitter, Portland Police declared a riot is taking place in the city and ordered crowds to go home.
"Disperse now or you will be subject to gas, projectiles, and other means necessary for dispersal," police said early on Saturday morning.
Portland is one of more than 20 cities across the US which saw protests on Friday night in the wake of George Floyd's death.
Earlier Portland police said that there had been "significant vandalism" in the city related to the protests, as well as a shooting, although they didn't provide any additional details.
"This event has been declared an unlawful assembly. If you do not go home now, force will be used to disperse you," Portland police said on their official Twitter.
According to police, Portland's Justice Center had been attacked by protesters and set on fire.
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