Witnesses told The LA Times someone ran into the bar about 11.30pm, U.S. time, and started firing what appeared to be a black pistol.
Authorities say there were multiple injuries - including one officer - after a man opened fire in Southern California bar late Wednesday.
Police are responding to reports of a mass shooting at the Borderline Bar & Grill during a College Country Nights event.
A sheriff's captain said at least six people were shot when a suspect opened fire.
Ventura County Fire Department said there was an "ongoing active shooter incident" reported at Borderline bar and grill. Information on the extent of the victims' injuries has not yet been released.
Thousand Oaks is located in Ventura County, which is just west of Los Angeles.
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We had moments, we had kind of nearly-chances, the biggest in the last two three minutes and before that not enough. The second half was really dominant, but we didn't score.
He said the man set off smoke grenades inside before opening fire.
The Times reports that there were multiple reports of shots fired and people hit.
A police helicopter swirled overhead and officers surrounded the area with cars.
He told reporters: "When our deputies arrive on the scene they attempted to neutralise that threat, I can't tell you if it was neutralised or not".
Witness John Hedge told ABC's 13WLOS TV station that said he saw a suspect throwing smoke bombs into the front of the restaurant.
Authorities are still treating it as an active shooter scene.
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