HALF MOON BAY, Calif. - Despite big wave warnings, two walls of water caught dozens of spectators off-guard and knocked them to a rocky beach as they watched a Northern California surfing contest Saturday, leaving some with broken bones but sparing them from being pulled into the ocean.

Thirteen people swept from a seawall had significant injuries, including broken legs and hands, said California Department of Forestry and Fire Protection Battalion Chief Scott Jalbert. He reduced the official count from the 15 reported earlier in the day but said others treated at the beach for injuries such as scrapes and bruises were not included in the total. At least three of the injured were taken to hospitals.

Jalbert estimated "a couple hundred" people were on the seawall at the southern tip of Mavericks Beach when the waves struck, upstaging the surfing competition that draws some of the world's top surfers.

The waves were 5 feet to 6 feet high by the time they hit - "small but strong," he said.

"Nobody was swept away into the water. They were just swept onto the beach area pretty hard," Jalbert said. "It's pretty rocky."

Additional firefighters had been on the way to clear the beach because of dangerous conditions but arrived too late, he said.

Only after the unexpected large waves swept in during high tide did the National Weather Service post a high surf warning until 10 p.m. Saturday. The agency previously posted a less severe high surf advisory.

"It's a force of nature that can't be predicted," Jalbert said. "We were very lucky that nobody was swept out to sea."

The surfing contest offers a $150,000 purse, making it the most lucrative big-wave contest in the world, even though it is held only when conditions are prime.

Competitors voted to schedule it because forecasts called for record-breaking tall waves, despite warnings that strong winds could make those breakers dangerously unpredictable.

Chris Bertish, who traveled 21 hours from his South Africa home, won the contest. He had just 48 hours' notice that huge waves were breaking off the coast and the Mavericks Surf Contest was on. Second place went to Shane Desmond of Santa Cruz.

The other winners were: Anthony Tashnick, Santa Cruz, third place; Dave Wessel, Kailua, Hawaii, fourth; Carlos Burke, Burle, Brazil, fifth; and Kenny Collins, Santa Cruz, sixth.

The two surprise waves knocked out barricades, a spectator platform and a large scaffold holding speakers broadcasting the contest, held in this tiny harbor town 25 miles south of San Francisco along Highway 1.

Marsha Poulin, of nearby El Granada, was at the water's edge minutes before the first rogue wave struck. She said she was concerned that organizers were letting spectators get so close to the ocean, given the conditions.

"Just because they were letting us be here doesn't mean it was safe," said Poulin, who left for higher ground just in time.

Brandon Snider, who got his injured knee taped up by a contest volunteer, said everyone was concentrating on the contest.

"It just came out of nowhere and wiped us all out," said Pamela Massette of Corte Madera. Her left hand and left knee were scraped and bleeding and she was wet from head to toe.

At least two more rogue waves came through the same area during the high tide, reaching the seawall, knocking down more spectators and sending others fleeing in panic. The subsequent waves were not as large or strong as the first two and did not cause any apparent additional injuries.

Spectators lost cameras, cell phones and backpacks as the waves swept the seawall.

Authorities yelled for people to get back from the shore after the waves struck but could not use the public address system because it had been swept away.

Officials were not allowing any new spectators to reach the beach, though they were allowing those already there to stay.

Bystanders from the seawall were moved about 100 yards back from the water after the chaos.

Two volunteers posted a makeshift leader board.

"This is probably the biggest and best contest we've had," said Ion Banner, 40, of Half Moon Bay, who was eliminated from the competition Saturday morning.

"It was consistently bombing."

He and fellow competitor Tim West estimated the waves were 20 to 25 feet measured from the back, with 40-foot faces.

"It was amazing out there. It definitely lives up to the hype," said West, 29, of Half Moon Bay, after he was eliminated from his first Mavericks contest.

The competition included 24 world-class surfers.

"If they did Mavericks with little waves, it wouldn't be Mavericks," said Diana Henderson, a National Weather Service Forecaster in nearby Monterey.

Associated Press Writer Don Thompson contributed to this report from Sacramento.