STANFORD, Calif. -- Police in riot gear were pelted by rocks, eggs and bottles Saturday as they tried to prevent violence that has marred the classic California-Stanford football game in recent years.

Twenty California fans were arrested after the game, apparently for refusing to leave the stands in an orderly fashion, said Stanford spokesman Jack Hubbard.

Stanford ensured its place in the Rose Bowl for the first time since 1972 with a 31-13 victory over California, and secured its first undisputed Pac-10 title.

The officers, clad in visored helmets, were attacked by Cal fans as they stood on the sidelines from the third quarter on, Hubbard said Saturday night.

The police presence, while controversial, was part of the school's efforts to make sure no one rushed the field after the Big Game, he said.

"There was a lot of concern about this, and I said, 'We want to send a message,"' Hubbard said. "We want a safe game. We do not want people on the field, because the minute that happens people get hurt."

Last year, three police officers were injured after fans charged the field following Stanford's 10-3 victory over Cal in Berkeley.

And in 1996, fans rushed the field and attacked the Tree, the Stanford mascot. Fans tore the tree costume apart, and Chris Carey -- who was inside -- suffered minor injuries.

The 100-year-old rivalry is reputed to be the oldest on the West Coast, and Stanford's victory was the Cardinal's fifth straight in the series.

Gary Migdal, Stanford's sports information director, characterized this year's disturbances as "extremely minimal."

"Twenty arrests for a crowd of 80,000 is nothing major," he said.