PORTLAND, Ore. — Protesters in Portland repeatedly set fire late Monday night to a police union headquarters building and were repelled by officers spraying tear gas, officials said. Twenty three people were arrested amid clashes that stretched into Tuesday morning.

The protesters in a group of about 300 people hurled rocks and bottles at officers and set three fires to the sides of the building and one to an awning, police said in a statement. All were put out.

One demonstrator climbed atop the building’s roof to unfurl a banner and the building’s walls were spray-painted, The Oregonian/OregonLive reported.

Portland has been gripped by nightly protests for months since the police killing of George Floyd in Minneapolis. Demonstrations, often violent, have targeted police buildings and federal buildings.

Before dawn Monday, police used tear gas to scatter demonstrators who hurled rocks, bottles and commercial-grade fireworks at officers and set fires in the streets as they marched on a precinct station in another night of violence. Twenty-three people were arrested.

During that demonstration, a group of about 200 lit a dumpster on fire and used it as a shield as they approached the precinct station, police said. They threw objects at officers, hitting some positioned on the station’s roof, and lit an awning on fire. One officer was left with a wrist injury after he was hit with a piece of ceramic.

The FBI said Monday that a threat last week that prompted the closure of a federal courthouse and other federal buildings did not appear to be credible.

Among the buildings closed was the Mark O. Hatfield U.S. Courthouse that was a target of protesters who repeatedly clashed in July with federal agents.