At least nine people were injured at a California college St. Patrick’s Day party when a garage roof collapsed Saturday.

Several thousand students gathered at California Polytechnic State University at San Luis Obispo, mostly wearing green, partying from Friday night into Saturday morning catching university and city officials off-guard.

About three dozen people were standing on the roof prior to it collapsing. Screams and gasps could be heard throughout the party as it came down.

The most serious injury of the collapsed occurred when a person’s thigh was impaled by a piece of debris from the roof, the city of San Luis Obispo said in a statement. Eight others were treated for cuts and bruises. None of the injuries suffered were life-threatening.

Three people were arrested for misdemeanors, Police Capt. Chris Staley said. Staley said the early partying caught police by surprise.

“This is completely unprecedented,” Staled told the San Luis Obispo Tribune. “We’ve never had anything like this.”

The university says it is working with police, and it has opened its own investigation that could result in disciplinary measures.  

School spokesman Matt Lazier said the school's fraternities and sororities have been on social probation and forbidden from holding parties at their own houses or elsewhere since January because of reports of possible sexual assaults.  

"Once we have all the facts, Lazier said, "the university will respond swiftly and accordingly."

The Associated Press contributed to this report