Nate Diaz and Jorge Masvidal don't fight for another month, but the action was well underway at their press conference on Thursday ... with "Gamebred" getting in the middle of a violent scuffle.

It all went down in Anaheim just minutes ago ... shortly after Diaz stormed off the stage without doing a ceremonial face-off with his opponent.

It's unclear what sparked it all -- we're assuming trash talk -- but in a video of the fracas, Masvidal is shown slapping a man's hand down -- and then the fists started flying.

It appears both fighters' crews were doing the most of the damage to each other ... with one guy taking a tumble into the seats and knocking the entire row over.