OAKLAND, California (CNN) -- A former transit police officer charged with murder was released from custody Friday after posting a $3 million bail.

Video shows Johannes Mehserle shoot Oscar Grant III in the back as another officer knelt on him.

Dozens of demonstrators gathered in downtown Oakland to protest the release of Johannes Mehserle, 27, charged with killing an unarmed man on New Year's Day. iReport.com: Watch the protest

The former Bay Area Rapid Transit police officer's release funds came from unknown sources, Alameda officials told CNN.

Video taken by a bystander showed Mehserle pulling his gun and fatally shooting Oscar Grant III, 22, in the back as another officer kneeled on Grant.

Mehserle may have intended to draw and fire his Taser instead of his gun, according to a court filing by his attorney.

In January, protests turned violent after a judge decided to allow bail for Mehserle's release.

A preliminary hearing in the case is set for March 23, authorities said.

CNN's Jackie Castillo contributed to this report.

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