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.
Lauren Boushey, 20, a Western Washington junior, who was at the apartment complex party said it broke up around 9 p.m. and police officers politely asked people to go home.
"It was set up to be ... this really nice night and nothing reckless or ridiculous like it turned into," she said Sunday, noting that the party wasn't connected to the riots and that people lingered and a crowd started forming a block away.
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.
Boushey said she saw several people throwing bottles and beer cans at police officers and provoking them. She saw multiple officers get hit by bottles.
"They got this horrible ignorant mob mentality," she said. "It was so sad and disrespectful to watch."
She said there a large police presence during the melee and that at one point what she called "a riot tank" rolled through with police officers hanging off the sides and loudspeakers warning people to disperse.
Scanlon said that there were multiple arrests, but he didn't have an exact figure. And he said he didn't know whether there were any injuries.
Bellingham, home to Western Washington University, is located about 75 miles north of Seattle.
The university released a joint statement from WWU President Bruce Shepard and Western Associated Student Body President Carly Roberts on Sunday afternoon saying that of the three arrests made, none were Western students.
But they said that public safety officers will be reviewing surveillance videos and criminal charges may then follow. School officials will also review records and will enforce the student conduct code.
They also said they were thankful that students were among those who stepped up to clean up the area.