POLICE and helicopters swarmed to California’s Del Mar Racecourse after reports of a gunman opening fire shortly before a scheduled Ice Cube concert.

Gunfire was heard during a televised horse racing event about 7pm local time (11.30am AEST). Amid confused reports, police immediately set up a perimeter around the main entrance.

All facilities within the crowded venue were placed in lockdown.

Crazy audio from TVG of possible gunfire at @DelMarRacing. @10News is en route for coverage. Scary. pic.twitter.com/edz17sstzq — Ben Higgins (@BenHigginsSD) September 3, 2018

The San Diego County Sheriff’s Department confirms one person was “down” and has been taken to the hospital, but did not confirm the nature of their injuries. Officials did confirm that a deputy was involved in the shooting, but did not detail the extent of his wounds.

CBS8 reports witnesses as saying the crowd had been getting “restless” as it filed towards the Ice Cube concert scheduled to begin once the races were completed. A person reportedly pulled a gun and fired three times into the air as he attempted to push his way to the front of the queue.

From eating a fish taco to ducking and running for my life. I was about 100 ft from the gate when gunshots were fired at the Del Mar race track. pic.twitter.com/T9B6WA031w — Matthew Hawkins (@MMAHAWK21) September 3, 2018

The deputy then reportedly used a taser in an effort to restrain the gunman, but this failed. Witnesses say the gunman was killed in the ensuing struggle, though this is yet to be confirmed.

Del Mar Racetrack says the situation has been contained, and the concert will go ahead as planned.

Shooting at the Del Mar Race Track right before the @icecube concert. Looks like the shooter is the only casualty. @DelMarRacing pic.twitter.com/ieRckXhqUt — Hugo Montes (@619Hurricane) September 3, 2018

They're on lockdown in del Mar and my mom is still there and my cousin is stuck inside :( this is scary they're still shooting — gabby mariee (@ppunkybrewster) September 3, 2018