PORTLAND • Police in Portland declared a riot late on Monday and made multiple arrests of protesters, ordering them to disperse after fires were lit at the offices of the police association.

"Failure to leave now will subject you to arrest and/or the use of crowd control munitions including but not limited to tear gas and impact weapons," Portland police said on Twitter, adding that personnel were responding to the fire near the city's North Lombard Street and North Campbell Avenue area.

The police later said officers had to move in again to address other fires set by demonstrators and made many arrests in the aftermath.

The police used tear gas on protesters and made 23 arrests late on Sunday as well, saying a group of more than 200 demonstrators had lit fires and attacked officers with lasers, rocks and bottles in the city's North Precinct.

Demonstrations against racism and police brutality have swept the United States since the death in May of Mr George Floyd, a 46-year-old black man who died after a Minneapolis police officer knelt on his neck for nearly nine minutes.

Portland police said last week that they had declared riots 17 times between May 29 and Aug 19.

Meanwhile, arsonists set buildings ablaze and torched much of the black business district in a second night of unrest in the city of Kenosha, in Wisconsin, which was triggered by the wounding of a black man who was shot in the back by officers as his three young sons looked on.

Smoke billowed over central Kenosha after police in riot gear clashed with protesters who defied a dusk-to-dawn curfew on Monday night and into yesterday morning, blocks away from where police shot Mr Jacob Blake on Sunday.

Mr Blake, 29, survived the shooting, which was captured on video, and was in stable condition following surgery, his father said.

But the incident, the latest in a litany of cases to focus attention on police treatment of African Americans, unleashed outrage in the lakefront city of Kenosha, about 65km south of Milwaukee.

REUTERS