Police arrested dozens of people and used tear gas against hundreds of demonstrators in Portland late on Saturday.
The rally in the north-western US city was to mark 100 days since Black Lives Matter marches against racism and police brutality erupted across the country.
Officers quickly declared it a riot after protesters hurled Molotov cocktails at them, and they responded with a barrage of tear gas to disperse the crowd.
At least one person was injured by the fire bombs, police said.
Dozens of protesters were arrested in an ensuing game of cat and mouse through a residential district of east Portland.
“It was Day 100 for Black Lives Matter protests in Portland since George Floyd’s murder and they wanted to prevent us from marching,” said protester Jay, 20, her eyes red from the tear gas.
“It’s our constitutional right to be here and to express ourselves.”
Police said people were “engaging in tumultuous and violent conduct thereby intentionally or recklessly creating a grave risk of causing public alarm”.
“This is a riot. Police are giving announcements to disperse. People are throwing Molotov cocktails,” Portland Police said on Twitter.
The nightly protests were sparked by the death of George Floyd in police custody in Minneapolis in May.
Floyd’s death triggered similar protests nationwide, but it is in Portland – a city of 660,000 that is more than 75 per cent white – that activists have remained on the streets almost every night, demanding racial justice and police accountability.
President Donald Trump has cast the city as being under siege from “thugs” engaged in “domestic terrorism”, though the demonstrations have been mostly peaceful.
Tension escalated again on August 29 after a man identified as a supporter of a right-wing group was shot and killed.
The suspect in the shooting was himself shot and killed by police five days later.
