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", the 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 said later that officers had to move in again to address other fires set by demonstrators and made many arrests in the aftermath.
Police used tear gas on protesters and made 23 arrests late Sunday as well, saying a group of more than 200 demonstrators had lit fires and attacked officers with lasers, rocks and bottles in the US city's North Precinct.
Demonstrations against racism and police brutality have swept the United States since the death in May of George Floyd, a 46-year-old Black man who died after a Minneapolis police officer knelt on his neck for nearly nine minutes.
President Donald Trump's administration in July deployed federal forces to Portland to crack down on the protests.
On Friday, he denounced the demonstrations as "crazy" and said cities run by Democrats had descended into chaos. Portland Mayor Ted Wheeler is a Democrat.
Portland police said last week that they had declared riots 17 times between May 29 and August 19.
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