LOS ANGELES — At least two inmates were stabbed Friday in a "large scale riot" at a maximum security prison in California, where gunfire was used to quell the unrest, authorities said.

Around 150 inmates were involved in the disturbance that started mid-morning in the main exercise yard of the state prison in Sacramento, said the California Department of Corrections and Rehabilitations.

"Staff utilized less lethal rounds, pepper spray and one lethal round (warning shot) to quell the incident," it said in a statement, adding that the more seriously injured were taken to hospital.

"The majority of the inmates involved sustained minor injuries as a result of the incident. Two inmates were stabbed multiple times during the incident and are being treated at an outside hospital," it added.

The prison houses some 3,000 inmates and has a staff of over 1,700. Opened in 1986, it primarily houses maximum-security inmates serving long sentences and those who have proved to be management problems at other institutions.

Cash-strapped California has for some years had a problem with prison over-crowding: the western US state has some 148,000 inmates housed in 33 jails designed for some 80,000 people, according to its own figures.

Images of inmates stacked into triple-bunk beds packed into gymnasiums drew widespread criticism, although officials claim to have made progress in the last few years, after the state's prison population peaked at about 175,000.

Late last year California appealed to the US Supreme Court to annul a ruling that some 46,000 inmates should be freed within two years, warning that the released prisoners could pose a risk to public safety.

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