WWE heavyweight champion CM Punk is lashing back at Chris Brown ... claiming the singer isn't just a liar ... but a total "coward" who never truly "paid" for what he did to Rihanna.
Punk released a video statement moments ago ... in response to Brown's allegations that he's a steroid-abuser with an inadequate penis.
In his video, Punk starts out by saying he's " a proud life long drug free straight edge individual" ... and then spends the next minute BLASTING Brown.
"Chris and I come from different worlds. I don't have an assistant. I don't have a body guard. I don't have a manager or a PR person telling me to delete tweets like Chris does, and I don't hit women. Period."
He adds, "[Chris] has made the wrong accusations to the wrong person because I can defend myself."
Punk then challenges Brown to a no-holds barred fight for charity ... and says, "I will choke you out and I will make you feel as weak and as powerless and scared and alone as any woman who has the misfortune of knowing a sad cowardly little boy such as yourself."
Your move, Chris.