Protests have flared again in Portland overnight as demonstrators clashed with police on the streets of the city which after months of sometimes violent confrontations has become a focal point of the US presidential race.
Police used smoke grenades and pepper balls to control the crowd of protesters. Officers were also seen arresting people who refused to leave the area.
About 200-300 people gathered in the downtown to march to the apartment of Portland Mayor Ted Wheeler to demand his resignation. They were seen lighting wooden benches and plastic trash bins on fire along the march.
Portland Police later declared the gathering a riot after protesters set fire in an apartment building, and ordered crowds to disperse or risk facing arrest.
The area was then secured to allow firefighters to respond to the situation, police said.
Portland has seen nightly protests since the killing of George Floyd, a 46-year-old African-American man, in Minneapolis on May 25. In recent weeks, tensions between right- and left-wing groups in the city have roiled downtown.
Hundreds of demonstrators have been arrested so far by the police since the protests have begun.
The administration of US President Donald Trump in July deployed federal forces to Portland to crack down on the protests.
State police and law enforcement from neighbouring suburbs were sent to Portland on Monday as tensions mounted following a fatal weekend shooting in the midst of clashes between supporters of Trump and counter-protesters.
Trump has seized on civil disturbances in Portland and other cities to blame state and local Democratic leaders as he amplifies his calls for law and order ahead of the November 3 presidential election.
His opponent in the election, Democrat Joe Biden, on Monday said it was Trump himself that was helping to stoke the violence.