The transit police officer who shot and killed an unarmed black man on New Year's Day has pleaded not guilty to murder.

Johannes Mehserle entered the plea Thursday in a packed Alameda County courtroom with supporters and family members of both the officer and the victim, 22-year-old Oscar Grant.

Prosecutors say Mehserle was standing over Grant, who was lying facedown and restrained on a train platform, when the Bay Area Rapid Transit officer fired one shot into the man's back.

Cell phone videos of the shooting have gone viral on the Internet, sparking outrage by many Oakland residents and African American community leaders.

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