Twelve people, including a police officer, have been killed after a gunman opened fire in a southern California bar.

Sheriff Geoff Dean of Ventura Co. Sheriff’s Office has confirmed the shooter is among the dead.

The venue was hosting a country music event for college students and its possible several hundred young people were inside.

Sheriff Dean said the suspect is still unknown and an investigation is ongoing.

"I have nothing to believe there is any terrorism link but we will certainly look at that option," he said.

Police said a highway patrol officer and another policeman, Sergeant Ron Helus, entered the bar after hearing gunshots.

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As the entered, Sergeant Helus was struck multiple times with gunfire.

Additional units, including SWAT personnel arrived on the scene minutes later and found 11 victims and the deceased gunman.

Police described the scene as "horrific" and said there was blood everywhere.

The wounded police officer was taken to hospital, but later died of his injuries.

"Sergeant [Ron Helus] was on the force for 29 years, was looking to retire in the next year or so and has survived by his wife and son," Sheriff Dean said.

"It’s a horrific incident, it’s part of the horrors that are happening in our country and everyone and I think it’s impossible to put any logic or sense to the senseless," he added.

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Multiple victims have been taken to hospital with injuries.

The incident happened at the Borderline Bar and Grill in the town of Thousand Oaks, a quiet, upscale residential suburb of Los Angeles.

"I’ve been a cop for 41 years and I never thought I’ve seen the things around the country would happen but I’ve learned it doesn’t matter what community you’re in, it doesn’t matter how safe your community is, it can happen anywhere," said Sheriff Dean.

The Los Angeles Times quoted a law enforcement official as saying at least 30 shots had been fired.

An unnamed witness told the newspaper that someone ran into the bar around 11:30 pm and started shooting what looked to be a black pistol.

"He shot a lot, at least 30 times. I could still hear gunshots after everyone left," the Times quoted the man as saying.