The demonstration that took place when I was in Sofia was by forest workers and wasn't at all violent, but it was large. The demonstrators started from in front of Alexander Nevsky Cathedral and I saw them again congregated in front of the National Council. The march was festive in a way, but you would be advised to keep a good distance from police, who are never all that friendly in Bulgaria. The police were in formation with truncheons, which makes me suspect that protests can be violent. It is best to use your common sense, maintain a good distance and, if things get rowdy, avoid the area.
During the economical crises in 1996 strakes were quite a frequent event.
It still happens but very rarely. Now the labour unions had organised a couple of strikes to show that people are not happy with the social policy of the state.
So, I took this picture 3 days ago. I just heard the noise while I was sitting in the office. I opened my window and saw these people gathered in front of the Ministry of Justice. Not a very big crowd and there was enough policy force to secure the place.
In case that you ever come across to such event just walk around of the crowd, nothing will happen to you. Just do not try to provoke them. For example, taking pics from a close distance is not a good idea.