The gunman is still on the loose, reported ABC7 Eyewitness News, which showed images of the Borderline Bar and Grill, in Thousand Oaks, California, surrounded by dozens of police vehicles.
An unnamed witness told the newspaper that someone ran into the bar around 11.30pm (local time) and started shooting what looked to be a black pistol.
A sheriff's captain says eleven people were shot late Wednesday when a suspect opened fire in a crowded Southern California bar.
Officers at the scene told the newspaper that about 30 shots had been fired.
"It was just some lowlife taking lives he shouldn't be taking", he said.
One witness saw the suspect tossing smoke bombs in front of the restaurant before watching a security guard get shot.
A trail of blood was visible on the street at the entrance of the venue.
Firefighters and first responders have arrived on scene and the Ventura County Sheriff's Office is working to secure the scene.
Police have described it as a "mass casualty incident" and helicopters are swirling overhead.
He said he heard from people inside that they were hiding in bathrooms and the attic of the bar.
People say he threw "smoke grenades all over the place".
Further information on the extent of the victims' injuries has not been released.
Authorities have urged the public to avoid the area and that roads are being blocked.
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