Partygoers were allowed to leave when the campus reopened. Photo:BBC

Campus news outlet Neon Tommy said the incident took place at 23:30 local time (06:30 GMT). Police confirmed that there were casualties.

Two suspects have been detained.

At least one suspect was briefly at large, but the university says the incident is over and the campus open. No students or staff were involved.

It had shut down the campus and urged students to stay away from the area immediately after the shooting.

Witnesses reported a "mob of people" fleeing from the main campus building.

'Screaming and running'

Neon Tommy contributor Matt Hamilton told the website he was among crowds of people trying to get into the party when he heard at least four shots in quick succession.

He said police came quickly, as there were already patrols on the campus.

"Everyone started running. I saw a guy who was shot in the leg. He was limping and screaming for his friends to help him," an unnamed witness said, quoted by the Los Angeles Times newspaper.

"People were screaming and running away."

USC's Department of Public Safety issued alerts asking students to stay away from the area, stay indoors and avoid opening the door to strangers.

Three people were slightly injured and a fourth received critical wounds to the buttocks and stomach, Neon Tommy said.

In 2008, USC sprinter Bryshon Nellum was shot in the leg three times at a Halloween party just off the campus.

He recovered and won a silver medal in the 4 x 400m relay at the London 2012 Olympics.

The latest shooting comes just months after 12 people were killed and 58 injured in a July shooting at a Batman film screening in Aurora, Colorado.