Odd Future -- Accused of Brutal Onstage Beatdown

Several members of the rap group Odd Future unleashed an unholy beatdown on a fan who dared to jump on stage during a show -- a fight captured on video -- and now, the fan is gearing up to sue.

17-year-old Chassan Rasagi filed a police report with the San Antonio PD, claiming they beat the ever-lovin' hell out of him.

Sources tell us ... OF rapper Hodgy Beats pushed Rasagi back into the crowd ...and he then became a human ping pong ball when the crowd pushed Rasagi back on stage. At that point, we're told the group attacked Rasagi en masse.

Rasagi suffered lacerations on his head, scratches, and burns to his body during the melee. We're told he also suffered two seizures. We're told Rasagi now plans to sue the group. It's worth noting some people in the crowd claim Rasagi threw the first punch.