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.