"Failure to comply with order may subject you to arrest or citation, and may subject you to the use of riot control agents or impact weapons," Portland Police announced on Twitter and on a loudspeaker at the scene on Saturday.
The rally was organised by Patriot Prayer, a far-right group founded by Joey Gibson, who is running for US Senate in Washington, reports Xinhua news agency.
The demonstration drew hundreds of Patriot Prayer supporters calling for "free speech" at a waterfront park around mid-day in downtown Portland.
Many of the right-wing demonstrators wore helmets and homemade riot gears while another group of counter-protesters from a coalition of labour unions, advocates for immigrant rights and other groups faced them from across the street, holding banners and signs.
The demonstrations, which appeared peaceful at the beginning, turned chaotic when some protesters clashed with the police.
The police said some protesters were throwing rocks, bottles and other projectiles at officers.
They ordered the demonstrators to disperse and leave the downtown area.
There was a heavy police presence, including bomb-sniffing dogs and weapons screening checkpoints, at the scene of the rally.
Saturday's event was the third rally staged by the Patriot Prayer this year in Portland to advocate "freedom of speech".
Two previous rallies had led to violent fights and riots.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
