(CNN) -- About 50 inmates rioted in the maximum-security yard at the California State Prison in Sacramento Wednesday afternoon, a prison spokeswoman said.
Guards used pepper spray and rubber bullets to contain the violence, which ended with at least nine inmates suffering stab wounds, gunshot wounds and blunt force trauma, according to California Department of Corrections and Rehabilitation spokeswoman Terry Thornton.
The injured were taken by ambulance to Sacramento-area hospitals, but their conditions were not immediately known, Thornton said.
A preliminary report said some guards suffered minor injuries, she said.
The 2,800 prisoners housed there are mostly "maximum-security inmates serving long sentences and those who have proved to be management problems at other institutions," she said.
CNN's Amanda Watts contributed to this report.