The assault happened around 11:15 a.m. Thursday in the area of First and Euclid streets, according to the Santa Ana Police Department.
As vehicles pass by around them, footage shows a man repeatedly punch and kick another man who is on the ground.
Police say several witnesses reported two men fighting, but both were gone by the time officers arrived.
It's unclear what may have led to the fight.
Video of the attack was posted on the Citizen app. Further details on the incident were unavailable.
Anyone with information is asked to contact police.