Sheriff's officials confirmed there was an officer-involved shooting but would not immediately provide details.
The sheriff's office said the suspect was "down" and had been taken to hospital.
At least one video posted on Twitter showed a female sheriff's deputy pull her gun and fire what sounded like four rounds after a man standing near an entrance gate is seen pointing a weapon toward the sky. A man in a black and white uniform with a walkie-talkie on his belt was then shown trying to restrain another man.
The concert, for which tickets were sold out, was scheduled to follow a full slate of horse races at the track, the station reported.
Commentators could be heard screaming "there's gunfire at the track" as the race took the place.
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Just before 6:40 p.m., a man came to the ticket window at the Del Mar Fairgrounds at 2260 Jimmy Durante Boulevard.
A shooting reportedly occurred Sunday in Del Mar, CA during the final lap of a horse race. and shortly before Ice Cube was set to take the stage there.
"An argument ensued at the gate and nearby deputies responded".
The suspect was transported to Scripps Memorial Hospital La Jolla and his condition is unknown at this time. Passing concert-goers can be heard shouting expletives after the shooting.
This article was originally published at 7:10 p.m.