Corrections officials said the episode at Calipatria State Prison began in the exercise yard about 6:30 p.m. Saturday when the one inmate began stabbing the others with a homemade weapon. The group got him to the ground and started beating him.
Ultimately about 115 prisoners were involved, the Los Angeles Times reported. Guards fired a warning shot with a rifle, and then used foam-tipped bullets and pepper spray to get the inmates back under control, the newspaper said.
Nine inmates were taken to area hospitals with "moderate to serious injuries," a Corrections Department statement said. No staff was hurt.
A second disturbance started later in a dining facility and kitchen, and lasted about 7 minutes.
Prison officials said the facility, located in Imperial County and home to 3,500 inmates, was placed on a "modified program" that limits inmate movement while the riot is investigated.
About 1,700 of the prisoners are serving life sentences, including nearly 600 who have no chance of parole.