Police responded to multiple reports of the shooting and Ventura County Sheriff's Office Captain Garo Kuredjian said a sheriff's deputy was among those shot.
Around 30 shots were fired, officers at the scene claim.
A witness told KABC-TV that there were many young people "having a great time" in the bar when a man entered and "started shooting".
Witnesses say a man fired several shots from a handgun before tossing smoke bombs into the crowded bar. "These poeple have never hurt anyone in their lives, they are just kids", he said.
It is thought the gunman may still be at large and at least police officer is injured.More news: Dems Gain in Quest for House Control But GOP Retains Senate
At least 11 people have been shot in a California bar hosting a college country music night.
Pictures and video footage show firefighters and first responders flooding the scene after multiple injuries were reported.
Police have described it as a "mass casualty incident" and helicopters are swirling overhead. Information on the extent of the victims' injuries has not yet been released.
Reports say the suspect may have used smoke grenades as well as a gun in the attack. An event aimed at university students was scheduled to take place this evening.
Authorities have urged the public to avoid the area and that roads are being blocked.