The fight at the California Rehabilitation Center in Norco involved 10 to 15 men, Luis Patino of the California Department of Corrections and Rehabilitation told the Los Angeles Times.
Officers quickly broke up the fight, he said. The inmates with the worst injuries were taken to hospitals, some with puncture and stab wounds, he said. Others were treated at the prison for small cuts and bruises.
Patino said it was not clear what set off the brawl. Those involved were transferred to the California Institution for Men in Chino, Patino said.
The Norco center was put under lockdown while the incident was investigated.