LEWISTON — An alcohol-fueled party for college seniors turned rowdy early Wednesday, leading to 11 arrests, an officer with a broken a leg and complaints that police overreacted.

The gathering that began late Tuesday was part of a Bates College senior tradition called “return to your freshman dorm.”

Lewiston police were summoned by campus security after some of the 250 to 300 students would not get out of the way of an ambulance that had been called to care for two injured women, police said.

Officers used pepper spray after 100 to 150 people refused to disperse, said Police Chief Michael Bussiere. He said at least one officer used a baton.

“There’s a difference between use of force and excessive force,” Bussiere said.

During the frenzy of arrests, a police sergeant’s leg was broken in two places, Bussiere said. A Bates student, Samuel Guilford, was charged with aggravated assault.

Students accused police of being overly aggressive.

Paul Chiampa, a junior from Pembroke, Mass., said officers grabbed him by an arm while trying to get the partiers to disperse. He said he told an officer to “take it easy” and the next thing he knew a half-dozen officers were wrestling him to the ground.

“It was absurdly excessive,” Chiampa said Wednesday as he sat in a dining hall with his elbow on ice. He had scrapes on a cheek and the bridge of his nose.

One woman and 10 men, including Guilford and Chiampa, were arrested. Most of the charges were for failure to disperse, disorderly conduct and refusing to submit to arrest.

Wednesday morning, the ground outside the dorm was littered beer cans, liquor bottles and other debris.

About 100 students gathered Wednesday afternoon in the quad to protest officers’ actions. Organizers asked students to sign a petition expressing objections to school administrators about what the students said was unnecessary force.

“Clearly, some students were out of line, but some police officers were also out of line,” said Joe Musso, a junior from Wallingford, Conn.

Tom Carey, director of security, said it was too early to say whether the criminal charges will affect any seniors’ participation in Sunday’s graduation.

The college, a selective liberal arts school with about 1,800 students, is conducting its own investigation into the episode, President Elaine Tuttle Hansen said in a prepared statement. She called the incident “highly unusual” for Bates.

“As a college community, Bates has enjoyed a long history of respect between our campus and local law enforcement,” she said.