Updated at 5:25 p.m. PST
Portland police say officers have arrested three people near Pioneer Square during a May Day rally and march which they now consider a riot.
Police said on Twitter Monday afternoon that anarchists destroyed a police car, damaged numerous windows and property, started fires in the streets and attacked police.
Police were telling people to leave or risk arrest after canceling a permit obtained for the May Day event.
It wasn't immediately clear if anyone had been injured.