Kelly Thomas' family told Eyewitness News that he was taken off life support at UCI Medical Center at noon Sunday.
Officers confronted Thomas on Tuesday night after receiving a call about someone breaking into cars.
When they tried to arrest him, Thomas fought back. Police said the brawl continued until Thomas lost consciousness.
Thomas' father said his son's face was almost unrecognizable.
Police said two officers suffered broken bones during the fight and were treated at a hospital.
Relatives say Thomas was homeless and had a history of mental illness, but they accuse the police department of using excessive force.
Court records show Thomas plead guilty to assault with a deadly weapon more than 16 years ago.
The Fullerton Police Department is investigating the incident.