A gunman armed with a pistol shot a bouncer and a cashier before detonating smoke bombs and opening fire on a crowded Southern California bar, according to terrified witnesses.

There are reports of multiple fatalities and police say the gunman is also dead.

Authorities say at least 11 people, including a sheriff's deputy, have been injured in the shooting tonight at Borderline Bar & Grill in Thousand Oaks, about 40 miles west of Los Angeles.

Hundreds were inside at 11.20pm local time (8.20pm NZT) when a man opened fire inside the bar, according to initial reports. Shots were still being fired when deputies arrived.

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Officials said there have been fatalities but could not say how many, the LA Times is reporting.

Nick Steinwender, student body president at nearby California Lutheran University, rushed to the scene when he learned of a shooting at the bar where he knew friends and fellow students were inside.

"It was chaos, people jumping out of windows, people hopping over gates to get out," Steinwender told KABC-TV.

He said he heard from people inside that they were hiding in bathrooms and the attic of the bar.

Ventura County Sheriff's Office Captain Garo Kuredjian told a news conference 11 people were injured, including a sheriff's deputy who had been taken to hospital.

However a Ventura County Sheriff Deputy spokesperson said: "That number is going to go up."

Authorities were earlier treating it as an 'active scene', though a police spokesman has now confirmed the gunman is dead inside the bar.

Nellie Wong, who was also celebrating her 21st birthday, said she scraped her knees diving to the floor when the shots rang out. Wong said she believed the gunman, who was dressed in a black hoodie, black shirt and black pants with a scarf obscuring his face, threw smoke bombs as he entered.

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"Thank goodness he didn't see me at all," she told the LA Times.

"I immediately stopped moving, stopped breathing. My heart was pumping really fast."

When police officers arrived, they exchanged gunfire with a suspect and one of the deputies was wounded, the LA Times reported. Kuredjian said he did not know if the gunman had been hit.

The FBI's Joint Terrorism Task Force had been dispatched to the scene.

He could not immediately provide information on the extent of the victims' injuries.

Responding deputies could hear gunshots as they arrived on scene.

Police told the LA Times that at least 30 shots were fired.

Borderline is a country music bar that hosts live music events. An event aimed at college students, which was open to students 18 and older was scheduled to take place this evening, according to the bar's website and Instagram page.

Police tell the Los Angeles Times that at least 30 shots were fired at the Borderline Bar & Grill in Thousand Oaks, which is about 40 miles west of Los Angeles. Photo / Google
Police tell the Los Angeles Times that at least 30 shots were fired at the Borderline Bar & Grill in Thousand Oaks, which is about 40 miles west of Los Angeles. Photo / Google

Carl Edgar, a 24-year-old Tarzana resident, told the LA Times he had about 20 friends inside the bar. He wasn't in the bar when the shooting occurred but was speaking with friends who were inside.

One texted him that she was hiding in a bathroom, fearing for her life. But Edgar said he was hopeful that his friends were safe.

"There are a few people we can't get a hold of, but in these situations people usually turn off their phones to be safe so I'm not gonna get too worried," Edgar said.

"A lot of my friends survived Route 91," he said, referring to a mass shooting that killed 57 during a country music festival in Las Vegas last year. "If they survived that, they will survive this."

Tayler Whitler, 19, said she was inside the bar when a gunman walked in with his face partly covered by something resembling a ski mask, opened fire on a person working the door, then began to shoot people at random."It was really, really really shocking," Whitler told KABC-TV as she stood with her father in the Borderline parking lot.

"It looked like he knew what he was doing."

Authorities have urged the public to avoid the area.

- AP and Daily Telegraph UK