Horrified parents have taken their children out of a California school at the centre of a gang rape attack, saying it is no longer safe.

The gang rape and beating of a 15-year-old girl in the grounds of Richmond High School after a weekend homecoming dance has shocked the community in one of America's most dangerous cities.

But even more horrifying, police said, was that up to two dozen people watched and did nothing to stop the attack. Some of them allegedly recorded the savage attack on their mobile phones and put a video on the internet.

One senior police officer in the crime-ridden city of 120,000 in the San Francisco Bay area called it one of the most heinous crimes he had seen.

Some pupils have already left the school in response to the attack.

"It's not safe there at all," said 16-year-old Jennie Steinberg, whose mother let her transfer out of the school yesterday. "I'm not going back."

The victim, a sophomore, had left the dance and was drinking alcohol in a school courtyard with a group when she was attacked, police said.

Two suspects were arrested, but police said as many as seven aged from 15 to their mid-20s attacked the girl for more than two hours in a dimly lit area near benches on Saturday night.

As many as 24 people saw the rape without notifying police.

Officers found the girl semi-conscious and naked from the waist down near a picnic table. She remained in hospital today.

"This was a barbaric act. I still cannot get my head around the fact that numerous people either watched, walked away or participated in her assault," Lt Mark Gagan said.

"It's one of the most disturbing crimes in my 15 years as a police officer."

Lt Gagan would not comment on rumours that observers recorded the attack on mobile phones and may have posted videos online.

Manuel Ortega, a 19-year-old former student, was arrested after trying to flee the scene. He is being held on £490,000 bail on suspicion of rape and robbery.

A 15-year-old pupil was also was arrested on Monday night for sexual assault, Lt Gagan said.

Last night SWAT teams were preparing to make more arrests as police offered a £12,000 reward for information.

"I'm confident that and at the end of our investigation we will get a clear indication of who was there and who did what," Lt Gagan said.

Lt Gagan said the girl left the dance and was walking to meet her father for a lift home when a classmate invited her to join a group drinking in the courtyard.

The girl had drunk a large amount of alcohol by the time the assault began, police said. Lt Gagan said the girl's father tried to call her mobile phone, but no-one answered.

Gagan said police received a tip about a possible assault on campus from a young woman who heard two men bragging about it. The girl was found nearly an hour after the dance, which more than 400 people attended, had ended.

The attack happened in a city that has dealt with its share of vicious crimes in recent years, and the school recently approved surveillance cameras after a series of violent crimes.

In one case a few years ago, a student was shot outside the school, ran inside and died in the then-principal's hands, said Marin Trujillo, a spokesman for the West Contra Costa Unified School District.

Richmond is an industrialised conclave near the San Francisco Bay that is known as one of the nation's most dangerous cities. In 2007, Richmond had 47 murders and the murder rate led the state for cities with populations of 100,000 or more, surpassing Los Angeles and Oakland.

That number dropped to 27 in 2008, but has increased to 44 killings so far this year, amid drug dealing and gang activity that has engulfed the town, Lt Gagan said.