Justin Bieber -- Sued for Allegedly Smacking Limo Driver

Justin Bieber has been sued by a limo driver who claims Bieber smacked him in the head 6 times because the guy wouldn't crank up the volume on the radio ... but the driver has a problem, and its all about race.

Abdul Mohar wants $850K from the Biebs for the December, 2013 incident in Toronto ... according to the Toronto Star.