Homeless Californian man, Kelly Thomas is beaten by Police, later dying in hospital.

A FAMILY is in shock after two police officers filmed in a brutal struggle with their mentally ill son were found not guilty of killing him.

Former Fullerton police Officer Manuel Ramos was acquitted of second-degree murder and involuntary manslaughter while Former Corporal Jay Cicinelli was acquitted of involuntary manslaughter and excessive use of force.

The two California-based police officers were seen by a jury on a 33-minute surveillance video where they struggled with Kelly Thomas.

Thomas was struggling with six police officers, who hit, kneed and jolted him with an electric stun gun as he was on the ground, calling out for his father repeatedly.

Those in the courtroom expressed their shock at the not guilty verdicts by gasping when they were read out.

Cicinelli, 41, wiped tears from his eyes and Ramos, 39, put his head down and rubbed his temples before turning to his supporters and winking, Fox News reports.

Thomas' father sat stone-faced while his mother wept.

Orange County District Attorney Tony Rackauckus, who tried the case himself, said after the verdicts that charges will be dropped against Joseph Wolfe, a third officer awaiting trial.

Thomas' parents condemned the verdict outside court.

"Just horrified," Cathy Thomas said. "He got away with murdering my son."

Ron Thomas said the verdict gave police "carte blanche" to attack people.

"All of us need to be very afraid now," he said. "Police officers everywhere can beat us, kill us, whatever they want, but it has been proven right here today they'll get away with it.

"Inside, we're all just torn up ... It hurts, it hurts really, really bad," he said."

The defence said Thomas started the confrontation by refusing to heed police orders and was fighting officers so much that they called for backup multiple times. At one point, the lawyers said, Thomas tried to reach for Cicinelli's stun gun.

Ron Thomas has countered that his son suffered from schizophrenia and didn't understand the officers.