Fifty-five prisoners have been taken to hospital and 200 others left with minor injuries following a riot and a fire at a jail in the southern California.
The riot broke out on Saturday night at the California Institute for Men in the town of Chino, east of Los Angeles.
A prison spokesman said 80 officers had responded to the unrest, which involved some 1,300 inmates in seven housing units and lasted about four hours.
Some of the injuries are considered life-threatening. No staff were hurt.
The prison spokesman said officers had used batons, pepper spray and unspecified "lethal force options" to bring the housing units under control by 0700 on Sunday.
"It has been reported that significant property damage was sustained, including one housing unit that sustained extensive damage due to fire," he said.
An investigation has begun to discover the cause of the fighting, he added.
Following the incident, all prisons in southern California were placed on lock-down and all visits suspended until further notice.
California has one of the most overcrowded and underfunded prison systems in the United States.
Last week, federal judges ordered state authorities to develop a plan to free within two years 40,000 of the 150,000 inmates being held.
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