Floyd Mayweather -- Sued Over Head-On Collision ... After Bieber Rescued His Kids

Floyd Mayweather is being sued over a frightening head-on collision involving his kids -- and Justin Bieber -- even though the boxer was on a red carpet when the accident went down.

We broke the story ... Floyd's kids were in a vehicle that slammed into another car in L.A. back in June while Floyd was at the BET Awards. Justin Bieber happened to be in the area at the time of the crash and rushed to the scene to comfort Floyd's kids.

Now, we've learned ... the other driver says he was seriously injured in the wreck and is suing Floyd ... claiming the man behind the wheel of Floyd's car was driving like a complete maniac that day.

And get this ... according to the suit, Floyd's driver is a FAMOUS BOXER -- J'Leon Love -- who trains out of the Mayweather Boxing Gym in Las Vegas. Love is also being sued.

According to the suit, Love was tearing through the streets at speeds of more than 100 MPH while weaving in and out of traffic.

The man behind the suit, Peter Park, claims he suffered injuries to his head, neck, collarbone, foot and ankle. Park claims his passenger was also injured.

Park is suing for unspecified damages. Calls to Love and Mayweather have not been returned.