A former California transit officer who fatally shot an unarmed man on an Oakland train platform in 2009 urged an appeals court Wednesday to overturn his involuntary manslaughter conviction, saying that letting it stand would place police under an increased threat of prosecution for making mistakes.

Johannes Mehserle, who worked for the Bay Area Rapid Transit, has long said he mistakenly drew his gun instead of his Taser when he shot 22-year-old Oscar Grant to death during an early morning melee on New Year's Day.

The shooting ignited civil unrest after it was captured on video and images of the white Mehserle shooting the black Grant in the back were repeatedly broadcast on television and the Internet.

On Wednesday, Mehserle's lawyer Dylan Schaffer said the shooting was a tragic mistake but not involuntary manslaughter, as decided by a jury in May 2010 after prosecutors sought a murder conviction.

Schaffer said such mistakes were usually resolved with lawsuits, and police officers need protection from criminal prosecution in such cases if they are to do their jobs properly.

"All he did was make an error," Schaffer said. "We cannot put them at risk for criminal sanctions for something that amounted to police error."

The lawyer said Mehserle's conviction will change policing in California unless the 1st District Court of Appeal tosses it out.

The three judges on the appellate panel asked few questions and revealed little about their thoughts on the case. Two of the judges did note that Mehserle used the Taser earlier that night and that it was located on the opposite side of his body as the gun.

Schaffer argued that tensions were boiling over during Mehserle interaction with Grant, clouding the officer's judgment.

"Officers were terrified people were carrying guns," Schaffer argued. "Lots of people in the Bay Area carry guns on New Year's Eve."

Schaffer also argued that the instructions the judge gave the jury before it began deliberations were flawed.

Mehserle was released from prison in June after serving half of a two-year sentence. He was in court Wednesday with his family and declined comment after the hearing.

Assistant Attorney General Gerald Engler said Mehserle's numerous mistakes and bad decisions that morning amounted to gross negligence and involuntary manslaughter.

Engler argued that Mehserle had no business using a Taser in the first place. He said that when Grant was shot, he was immobilized, with another officer kneeling on his neck, and was complying with police.

Engler also noted the Taser was in a separate holster on the opposite side as Mehserle's gun, requiring him to reach his right hand across his body to get it. Engler said the weapons also look and operate differently. The safeties have different mechanisms and the weapons are different colors and shapes.

Taken together, Engler said Mehserle's "mistakes were so numerous and so great" that the jury properly concluded he was guilty of involuntary manslaughter.