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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.