SAN QUENTIN, Calif. (AP) -- Forty-two inmates were injured Monday when a simmering dispute between two ethnic groups erupted into the largest riot at San Quentin State Prison in 23 years.

The fight broke out between white and Hispanic inmates in a medium security dormitory-style unit that houses roughly 900 prisoners, said Vernell Crittendon, the prison's public information officer.

Prison officials said as many as 80 inmates in several different buildings were involved in the tumult, which lasted six minutes. It took about 50 officers armed with batons and pepper spray to quell the fight, said Sgt. Eric Messick, a warden's administrative assistant.

Three seriously injured inmates were taken to area hospitals, authorities said.

"One inmate has a slash wound that goes from his ear all the way over to his mouth. It appears he was attacked by a razor blade attached to a toothbrush or a comb," Crittendon said.

Authorities discovered five weapons in the unit, including three soap bars stuffed into socks, a razor and a "small weapon made out of a metal tip, melted into the end of a pen," Messick said.

Authorities could not yet pinpoint what sparked the riot, though they ruled out gang or drug-related activity. An investigation was continuing.

The entire prison was put on lockdown early Monday and hundreds of bruised inmates were placed in plastic handcuffs as officers searched the unit and separated the groups.

Parts of the same unit of medium-security prisoners have been on lockdown since Aug. 2, when several small fistfights broke out between the same groups involved in Monday's riot, Messick said.

"This is an escalation of hostilities between the two groups," he said.

Crittendon said the last riot of this scale occurred in 1982, when 1,000 men rioted in the prison's upper yard.

Todd Slosek, a spokesman for the California Department of Corrections and Rehabilitation, said he was pleased prison staff could quell the conflict without using significant force.

"We did spray a lot of pepper spray," he said.

(Copyright 2005 by The Associated Press. All Rights Reserved.)