PORTLAND, Ore. (AP) — Protesters and Portland police clashed again in Oregon’s largest city late Monday, with demonstrators hurling rocks, punching a police sergeant in the face and spraying a chemical irritant at officers, police said in a statement. 24 people were arrested.

The demonstration with about 150 protesters took place outside a police union building that has turned into a frequent protest site, police said.

The protesters with makeshift shields gathered earlier for their demonstration in a park and officers moved in to take away the shields, police said.

A scuffle happened when officers tried to seize a sign that said “vote,” multiple videos from the scene showed, and police also used pepper spray, The Oregonian/Oregonian Live reported.

Police in their statement called the protesters’ walk from the park to the union building a “planned unpermitted march.”

Most of those detained amid clashes were arrested on suspicion of interfering with a police officer and disorderly conduct. The officer who was punched in the face was taken to a hospital for medical treatment, the police statement said.

Portland has been gripped by nightly protests for more than three months since the police killing of George Floyd in Minneapolis.

The demonstrations, often violent, have targeted police buildings and federal buildings.

Some protesters have called for reductions in police budgets while the city’s Democratic mayor and some in the Black community have decried the violence, calling it counterproductive.