Demonstrations are continuing in the US city that had hoped for calm after federal agents withdrew more than a week ago.
Three officers were hurt, including two who were taken to a hospital, during efforts to clear a crowd of several hundred people outside the Portland Police Association building late on Saturday. The two officers have since been released.
Rallies had been held earlier in the afternoon and evening throughout the city, including at Peninsula, Laurelhurst and Berrydale parks, local media reported.
Police said a group from Peninsula Park marched to the Portland Police Association building, which is located about 5 miles north of the federal courthouse that had been the target of nightly violence earlier this summer. The Portland Police Association is a labour union that represents members of the Portland Police Bureau.
A group of demonstrators broke into the building, set the fire and were adding to it when officers made the riot declaration just after 11.30pm, police said.
Video shot by a journalist, and surveillance video from inside the building obtained by the police department, shows smoke and flames arising from inside the building.
Officers formed a line and used flash bangs and smoke canisters to force the protest back by several streets. Demonstrators congregated at Kenton Park, where they were again ordered to disperse. Most of the crowd had left by 2am, police stated.
The gatherings this week had been noticeably smaller than the crowds of thousands who turned out nightly for about two weeks in July to protest against the presence of US agents sent by US President Donald Trump’s administration to protect the federal courthouse in the city centre.
This week’s clashes have, however, amped up tensions after an agreement between state and federal officials seemed to offer a brief reprieve.
Police arrested 24 people during demonstrations overnight on Friday after they said people defied orders to disperse and threw rocks, frozen or hard-boiled eggs and fireworks at officers. An unlawful assembly was declared outside the Penumbra Kelly public safety building.
Most of those arrested were from Portland, while one man was from Oakland, California, and another was from Tulsa, Oklahoma. Most were in their 20s or 30s. The charges included assault on an officer, interfering with an officer, disorderly conduct and resisting arrest.
Since George Floyd was killed in Minneapolis, protests over racial injustice and police brutality have occurred nightly for more than 70 days.
Democratic mayor Ted Wheeler said violent protesters are also serving as political “props” for Mr Trump in a divisive election season where the president is pushing a law-and-order message. Mr Trump has called the protesters “sick and dangerous anarchists”.
Tear gas was used by police on protesters on Wednesday for the first time since the US agents pulled back their presence in the city. But officers did not use it Thursday or Friday despite declaring the demonstrations unlawful assemblies. Police said tear gas was not used on Saturday.