Shannon Briggs is declaring war on boxing champ David Haye -- claiming the guy's a "sucka" who weaseled his way out of a heavyweight fight ... because he's scared.
Briggs says Haye PROMISED him a bout if he fought on the undercard of Haye's last fight -- so Briggs did (and won) ... and now he wants to collect.
Problem is, Haye's running ... and Briggs is calling him out.
"He's a SUCKA ... he's not a real fighter! He's going back on his word."
In fact, Briggs chased Haye down at the Carl Frampton fight in Brooklyn over the weekend -- but Haye couldn't even look him in the face to talk like a man.
Now, Briggs is calling out the ENTIRE heavyweight division -- saying he'll fight ANYONE, ANYTIME.