Multiple partiers were arrested during the melee late Saturday and early Sunday in Bellingham, according to police Sgt. Mike Scanlon.
"There was drinking, it became disorderly and pretty much an out and out riot," he told The Associated Press.
He said the unrest began as police dispersed a noisy party that had drawn a few hundred people.
Many of the revelers then moved to nearby Laurel Park, where they were joined by even more people, Scanlon said, calling it a "large, intoxicated, disorderly crowd."
The situation "finally boiled over," he said. 'They began hurling projectiles at police."
Up to 500 people had converged on the park as police worked to disperse them, at one point using pepper spray, Scanlon said.
It took about 45 minutes to clear the area and restore order.
Scanlon said that there were multiple arrests but he didn't have an exact figure. And he said he didn't know if there were any injuries.
Bellingham, home to Western Washington University, is located about 75 miles north of Seattle.