PORTLAND, Ore. (AP) — A fire was set to the outside of a police union building late Thursday, leading authorities to declare an unlawful assembly as Portland, Oregon, grappled with another night of protests over racial injustice.

Several photos and videos posted online showed flames erupting outside the doors of the Portland Police Association office. The fire was quickly extinguished and demonstrators were ordered to disperse, news outlets reported.

Portland police declared an unlawful assembly and tweeted that those who failed to leave could be subject to “arrest, citation, or the use of crowd control agents.”

On Wednesday night people hurled several firebombs at officers in Oregon’s largest city during a demonstration over a Kentucky grand jury’s decision to not indict officers in the fatal shooting of Breonna Taylor, police said.

The city has already seen nearly four months of nightly protests over racial injustice and police brutality.

Deputy Police Chief Chris Davis said Wednesday night’s demonstrations were the most violent that Portland has seen thus far in four months of nearly nightly unrest since the death of George Floyd, a Black man who died in Minnesota after a white officer held a knee to his neck. Multnomah County District Attorney Mike Schmidt — who has been criticized for dismissing cases against hundreds of protesters — condemned the violence and called for calm.

Thirteen people were arrested during the demonstration.

U.S. agents with the Federal Protective Service, who were guarding a federal courthouse nearby, offered assistance and the Portland police accepted because it was an “emergency need in the moment,” Davis said at a news conference Thursday.

The right-wing group Proud Boys plans a rally in Portland this Saturday to support President Donald Trump and the police.