LSU Tailgate Fight: Cops Investigating Casted Ass-Kicker

The LSU Police Dept. has opened an investigation into a crazy tailgate brawl on Saturday involving a man with ONE ARM IN A CAST who brutally punched the hell out of several people ... TMZ Sports has learned.

The video, shot by Zac Hall, went viral minutes after he posted it online -- showing a man with his right arm in a purple cast up to his bicep unloading on several LSU fans, knocking at least one guy out cold.

The victim appeared to be trying to diffuse the situation when the cast guy struck. The unidentified man proceeded to throw more punches at the victim and anyone else who tried to step in.

A rep for the LSUPD tells us, "No arrests at this time but LSU police are investigating."