Former NFL running back LeGarrette Blount was involved in a fight at a youth football game on Saturday ... throwing punches in a wild skirmish -- and now, TMZ Sports has learned cops are investigating it all.
The altercation happened following the conclusion of a 12-year-old-and-under game in Gilbert, Arizona.
Blount was coaching the GCYFC Gators against the Chandler Elite Bears -- when in the postgame handshake line ... he appeared to take exception to something someone had said to him.
Drone video shows Blount became upset -- and motioned animatedly toward coaches on the opposing team for several moments.