Well, the self-defense you refer to is precisely what I would consider a legitimate use of violence, indeed an inalienable human right. The Black Panthers are the example I remember; there are others that the Black Bloc theoretically emulates.

As to Gandhi, it seems this is not so simple. It seems that Gandhi's movement was the culmination of several hundred years of often brutally violent resistance to British rule (Edward Said hardly even mentions Gandhi in Culture and Imperialism and in fact views the focus on any small fraction of that time span as serving colonial propaganda interests). Even while Gandhi was active, his was not the only movement; this, it is argued, suggests that multiple tactics are often necessary. Finally, in many instances where the oppressor seems insensitive to bad publicity (as with the Occupy movement in the U.S.), nonviolence can be a form of complicity with ongoing violence by the regime--both physical as manifest in the suppression of Occupy, and structural as with the human rights-denying social inequality that, in the U.S., is the most vast in the high-income world.