Orange County District Attorney Tony Rackauckas said the hearing could take one to two days. A paramedic and a forensic specialist both testified Monday.
The prosecution displayed photos Monday of Thomas's bruised and bloodied face following the beating. They also showed photos of minor scrapes and cuts officers sustained during their encounter with Thomas, including Officer Manuel Ramos and Corporal Jay Cicinelli. They were the only two out of the six officers involved that night to be charged in the case. The incident prompted a federal civil-rights investigation, as well as weeks of protests by Fullerton residents.
Newly released surveillance video from the bus terminal showed Ramos and Cicinelli talking to Thomas as the situation began to escalate. The video and an image of a hospitalized Thomas provoked gasps from court spectators.
Ramos has pleaded not guilty to second-degree murder and involuntary manslaughter charges, and Cicinelli has also pleaded not guilty to involuntary manslaughter and excessive force under color of authority.
The incident happened July 5, 2011, and was caught on tape by a bystander. Prosecutors say the beating of Thomas began after officers responded to reports that a homeless person was looking into cars and rattling door handles.
Officers stopped Thomas and asked him to sit on a curb, but the 37-year-old, who was reportedly schizophrenic, had trouble complying. Prosecutors allege that's when Ramos punched Thomas in the ribs and tackled him to the ground and Cicinelli Tasered him several times and struck him in the face with the Taser. Thomas died five days later in a hospital after being taken off life support.
The court also saw Ramos's injuries to his arm and leg, and a thumb injury sustained by Cicinelli in the altercation.
The coroner concluded that Thomas died from mechanical compression of the thorax, which made it impossible for him to breathe normally and deprived his brain of oxygen. Other face and head injuries contributed to his death, prosecutors said.
A forensic specialist with the Fullerton Police Department said Ramos was holding his rib cage and was out of breath when she arrived at the scene of the incident. Appearing exhausted, he told her he had never had anyone fight him like that before, she testified.
The defense says no excessive force was used and the conduct was appropriate considering the circumstances.
Supporters of Thomas were both inside and outside the courthouse Monday. Thomas's father Ron says he wants justice for his son.
"You have 1,500 pounds of trained cop on my one little son, but they have to bring this out like they were just so overpowered by this brute to make themselves look innocent and that they're the victims," said Ron Thomas.
All six officers who responded that night were placed on administrative leave and face an internal investigation.
The Associated Press contributed to this report.