A gunman barged into a large, crowded Los Angeles-area country music bar and dance hall and opened fire, fatally wounding at least 11 people including a police officer, US police said.

Ventura County Sheriff's Office Sergeant Eric Buchow told the CBS affiliate in Los Angeles that there were "multiple fatalities" in the shooting and the gunman was also dead inside the crowded venue in the upscale residential suburb of Thousand Oaks.

Buschow did not say how the gunman died.

Authorities have not confirmed whether any of the wounded had died.

Ventura County sheriff's Captain Garo Kuredjian said hundreds of people were inside the Borderline Bar & Grill in Thousand Oaks at 23:20, and shots were still being fired when deputies arrived.

Several people from inside the bar told TV stations that a tall man wearing all black with a hood and his face partly covered first shot at a person working the door, then opened fire, seemingly at random, at the people inside.

People screamed and fled to all corners of the bar, while a few people threw barstools through the windows and helped dozens escape, witnesses said.

It was college night and country two-step lessons were being offered on Wednesday at the Borderline, according to its website.

Kuredjian said it has been "quite some time" since there was a shooting of any kind in Thousand Oaks, a city of about 130 000 people about 64km west of Los Angeles, just across the county line.

^{Authorities say there were multiple fatalities – the gunman including one officer. (Mark J. Terrill, AP)}

^{People comfort each other as they stand near the scene where a gunman opened fire inside a country dance bar. (Mark J. Terrill, AP)}

^{An FBI agent talks to a potential witness near the crime scene. (Mark J. Terrill, AP)}

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