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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.

5:16 PM PT -- Chris Brown responded:

