A Ventura County Sheriff's Office official said approximately 30 shots had been fired inside the building.
The venue's website says for a quarter of a century it "has stood as the Ventura County's largest country dance hall and live music venue".
In another Twitter post, the reporter put up a video of youngsters he said "were apparently in the Borderline bar" at the time of the reported mass shooting.
"The gunman was throwing smoke grenades all over the place", a different witness told KABC.
There were reports of more gunfire as officers responded.
One person was seen being taken out on a stretcher and placed in an ambulance, and one deputy was shot.
He said he heard from people inside that they were hiding in bathrooms and the attic of the bar.
Attorney General Jeff Sessions submits resignation letter
The president also warned the Democrats against investigating him, after taking over the House in the midterm elections . In that scenario, Mueller's budget could be reduced "so low that his investigation grinds to nearly a halt", Mr.
Authorities have urged the public to avoid the area and that roads are being blocked.
No other information was immediately available.
Authorities say there were multiple injuries - including one officer - after a man opened fire in Southern California bar late Wednesday. The extent of the victims' injuries wasn't immediately known.
"Firefighters and first responders are arriving on scene of a report of a shooting at an establishment in @CityofTO. I just started hearing these big pops", the witness, John Hedge, told the local ABC affiliate.
Witnesses at the Borderline Bar & Grill reported that a black-clad shooter hurled smoke bombs into the restaurant as he let loose a fusillade of shots.
CNN said people smashed windows with chairs to escape.
Police said that at least 30 shots had been fired.