The issue of rebelling against a Muslim ruler is extremely controversial and complex, and it is disastrously dangerous to engage in takfir even on the basis of great injustice and oppression. I would strongly discourage revolutionaries from following this path.
As I have written before, try to keep your focus and your fight against the Kuffar who, in actual fact, are the rulers of the imperial system which controls the individual repressive regimes in the Muslim world. The permissibility of fighting them is crystal clear; it is more effective, and safer for your imaan.