This is very difficult because the definition of "protest" is not a very concrete definition.
If protests are things like the ones in England, which were touched off by complaints about police actions in an essentially free and open society, violence cannot be justified. The same goes for protests against austerity measures in Greece and places like that. These things are, essentially, protests over issues that can and should be dealt with politically.
On the other hand, if we define things like what is going on in Syria as protests, violence becomes much more justifiable because these situations are more like rebellions against tyrants. The government there is one that shows no signs of being interested in working anything out politically. They are attacking protestors with great violence. In such cases, it seems that violence is justified because it is being deployed against a much greater evil and against a government which will not consent to political discussions.
So, as long as protests are about things that are not hugely evil, and as long as the governments involved would be willing to engage in a political dialogue with the protestors, violence is not justified. In cases like Syria, however, violence seems much more justifiable.