Fighting broke out in a prison yard about 10:45 a.m., said Margot Bach, a state corrections spokeswoman.
It was unclear how long it took to quell the riot but "the entire institution is on full lockdown," Bach said. The prison is in the Mojave Desert about 40 miles north of Los Angeles.
Guards fired several warning shots before using pepper spray, tear gas and wooden bullets on the prisoners, Bach said. She said it was unclear whether any prisoners were hit. No guards or staff were hurt.
Two inmates were stabbed and had collapsed lungs, while two others had serious slash wounds, she said. They were to be taken by helicopter to local hospitals.
Three other inmates were taken by ambulance to hospitals with unspecified injuries.
The prison was designed to hold 2,200 prisoners but currently has more than 4,000 inmates in its minimum- and maximum-security wings. It was not immediately clear whether crowding may have contributed to the rioting and Bach said it may take several days to determine the exact cause.
Last year, a racial brawl involving more than 120 inmates left 10 people injured. In 1999, an inmate was shot by guards during another brawl between whites and Hispanics.
Bach said race didn't appear to be behind Thursday's riot.
"It was multiracial. There wasn't any group attacking any other groups specifically. They all got into it," she said.