Justin Bieber -- Attacked Limo Driver from Behind ... Cops Say

Justin Bieber was charged with assault in Toronto because he allegedly bashed a limo driver in the back of the head.

Toronto Police have released new details of the Dec. 30 incident ... and say the limo driver had picked up 6 passengers around 2:50 AM -- including Justin -- when a fight broke out between the driver and "one of the passengers."

Cops say the passenger -- allegedly Justin -- hit the limo driver in the back of the head "several times" during the fight, and when the driver stopped to call police ... Justin fled the scene.

Bieber turned himself in last night -- what a scene -- to face the assault charge.

Bieber's lawyer, Howard Weitzman, says he expects this will be treated like a low level crime -- the equivalent of a misdemeanor is the U.S. And, Weitzman says, "Our position is Mr. Bieber is innocent."