Headbutts, punches, trash-talk ... yeah, all hell broke loose during a pre-fight clas between heavuweight champ Deontay Wilder and Artur Szpilka today -- and TMZ Sports has the footage.
It all went down at the World Trade Center -- when the two fighters got face-to-face and started yelling at each other.
That's when Szpilka let the hands fly -- and a scuffle broke out ... with a bunch of people flying in to break it up before the guys killed each other.
Obviously, these guys are trying to drum up publicity before they get in the ring for the big fight in Brooklyn on Saturday night -- but we're told from people close to the situation, the animosity is real .