I would say that undirected violence is always wrong -- If there's no link between the target and whatever circumstance has led to violent protest, then the violence is unjustified and only serves to generate fear. Although fear alone can has been shown to be an effective way to gain and hold power, I think we can agree that it is not the preferred path to victory, particularly for a movement that claims to be for good. It also tends to raise despots who may bend the momentum of the movement for their own benefit or goals after the original goal is achieved.
In my opinion, there's an escalation of response that is necessary to follow. People who have a voice in the government of their country have a responsibility to fight for change through the courts, at the voting booth, and using every peaceful means at their disposal. There can come a time, after it is clear that peaceful means are not just unheard, but ignored, when violence can become a reasonable response to the oppression. Up until that time, and continuing throughout whatever violent protest occurs, the movement must maintain course trying to effect change through the system as well; otherwise, they stand to lose whatever moral authority they have gained, and the story soon become about the physical fighting, rather than the cause.