An officer with the Los Angeles Police Department confirmed there had been a shooting on campus and said four people had been taken to hospital, but could not immediately provide further details.
USC said on its website that a suspect was in custody following the shooting, which occurred at 11:30 pm (0630 GMT) Wednesday.
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The campus emergency website urged students to stay away from the area, citing an "ongoing police investigation."
The LA Times reported that "several" people were shot at a Halloween party on the campus.
The private university in Los Angeles has more than 38,000 students and boasts the largest number of international students of any US university.
