A gunman has shot and killed 12 people in Southern California, using a handgun and smoke bombs at a country dance bar on student night.

Key points: Sheriff's sergeant Ron Helus was killed by the gunman when he responded to the attack

Sheriff's sergeant Ron Helus was killed by the gunman when he responded to the attack Survivors broke windows to escape the bullets

Survivors broke windows to escape the bullets Authorities have "no idea if there is a terrorism link"

Ventura County Sheriff Geoff Dean said a sheriff's deputy who was killed in the attack was one of the first to respond to calls for help, shortly after 11:20pm local time.

The officer, Ron Helus, died in hospital.

Sheriff Dean said around 10 other people were shot and injured.

"Multiple other victims of different levels of injury … were rescued from the scene and taken to local hospital," he said.

An FBI agent talks to a potential witness as they stand near the scene where a gunman opened fire inside a crowded country dance bar. ( AP: Mark J. Terrill )

"We have no idea if there is a terrorism link to this event or not."

Sheriff Dean described the scene as "horrific". No other information on the victims was immediately known.

"There's blood everywhere," he said.

The gunman was dead inside the bar, Ventura County Sheriff's Sergeant Eric Buschow said, but he did not say how he died.

Survivors and witnesses described the "shocking" scenes after the shooter opened fire. ( AP: Mark J. Terrill )

Sergeant Helus was a 29-year veteran of the force with a wife and son and planned to retire in the coming year, said the sheriff, who choked back tears several times as he talked about his longtime friend.

"Ron was a hardworking, dedicated sheriff's sergeant who was totally committed," he said.

"Tonight, as I told his wife, he died a hero because he went in to save lives."

The attack sent hundreds of panicking people toward the exits, with some breaking windows to escape, authorities and witnesses said.

People comfort each other near the scene where a gunman opened fire. ( AP: Mark J. Terrill )

Eyewitness Taylor Whittler, 19, said she and her friends were dancing when the gunman attacked.

"He was really fast so he knew what he was doing and he also had perfect form. When I look back he was like spot on," Mr Whittler told local media outlet KABC.

"It was really, really shocking … I never thought something like this would happen."

Nineteen-year-old Erika Sigman found herself hiding with a group of strangers, and they were holding her hands.

"There is a lot of bad in this world, but there is also a lot of good and people will help," she said.

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Ventura County Sheriff's Captain Garo Kuredjian said hundreds of people were inside the Borderline Bar & Grill in Thousand Oaks at the time of the attack, and shots were still being fired when deputies arrived.

Several people from inside the bar told TV stations that a tall man wearing all black with a hood and his face partly covered first shot at a person working the door, then opened fire — seemingly at random — at the people inside.

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People screamed and fled to all corners of the bar, while a few people threw barstools through the windows and helped dozens escape, witnesses said.

It was college night and country two-step lessons were being offered Wednesday at the Borderline, according to its website.

Captain Kuredjian said it has been "quite some time" since there was a shooting of any kind in Thousand Oaks, a city of about 130,000 people about 64 kilometres west of Los Angeles, just across the county line.

ABC7 Eyewitness News showed images of the bar surrounded by dozens of police vehicles, with victims being treated by paramedics.

There were reports of pools of blood in the area, and responders attending to one injured person outside the venue.

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A witness described to ABC7 Eyewitness News hearing at least 12 shots before he escaped out the door, with a security guard down on the floor injured.

He said the man set off smoke grenades inside before opening fire.

Another witness said he saw a security guard being shot as well as a female cashier, before he also fled.

"It was just some lowlife taking lives he shouldn't be taking," he said.

"It was just young people having a great time and this maniac came in and started shooting people for no reason at all."

The Borderline Bar and Grill in Thousand Oaks, California. ( Google Images )

ABC/AP