Police officers were hit by rocks and bottles during a protest in the Oregon city of Portland late on Saturday, police said, and there were clashes between rival groups of demonstrators in the city's downtown area earlier in the day.
In a Twitter post, police said early on Sunday they had declared a riot for the gathering around the Penumbra Kelly Building, a city building that houses offices including police.
"Failure to leave may subject you to arrest, citation, or crowd control agents, including but not limited to tear gas and/or impact weapons," the statement added.
There were scuffles on Saturday afternoon between anti-racism protesters and right-wing demonstrators.
Demonstrations against racism and police brutality swept the United States after the death in May of George Floyd, a 46-year-old Black man who died after a police officer knelt on his neck for nearly nine minutes.
President Donald Trump's administration in July deployed federal forces to deal with the nightly protests in Portland, where demonstrations have sometimes erupted into violence.
On Friday, Trump called Portland's demonstrations "crazy" as he said cities run by Democrats had become lawless and chaotic. Portland Mayor Ted Wheeler is a Democrat.
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