Law enforcement declared an unlawful assembly Friday night after protesters marched through the streets of Portland on to a police building, where officers stood waiting outside.
A few hundred demonstrators had met at Kenton Park before making their way to the Portland Police Association building, where police warned protesters to stay off the streets and private property.
Those who refused could be subject to citation, arrest, the use of tear gas, crowd-control agents or impact munitions, police said.
Around midnight, police ran down the street, pushing protesters out of the area, knocking people down and arresting those who they say were not following orders - as some people were being detained, they were pinned to the ground and blood could be seen marking the street pavement.
Demonstrations in Portland, which started in late May after the police killing of George Floyd in Minneapolis, are reaching 100 straight nights of protests that have been marked by vandalism and violence.