Authorities say multiple people - at least 11 - were injured in the shooting, including one officer, though the extent of the injuries wasn't immediately known.
Ventura County Sheriff's Office Captain Garo Kuredjian said it is "still an active scene".
Multiple people were seen being led out of the bar by deputies.
Police in the U.S. are responding to reports of a shooting at a bar in Thousand Oaks, California.
About 30 shots were fired with a semi-automatic weapon, the Ventura County Office and California Highway Patrol said.
One witness said there were multiple shots fired inside the bar.
Social media reports claim the suspect shot the bouncer before throwing a smoke grenade and firing more shots.
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The attack has been upgraded to a level 2 mass casualty incident indicating that there are between 10 to 20 injured victims.
There were reports of more gunfire as officers responded.
The status of the gunman has not yet been confirmed, but police said they did not believe there was a wider threat to the public.
Israel's Consul General in Los Angeles Sam Grundwerg released a statement that "I have not received any information on Israeli victims" in the incident.
Tonight's event was aimed at college students, according to the bar's website and Instagram page.
Borderline Bar & Grill is a western-style facility that hosts theme nights such as country, salsa and swing dancing.