Ask yourself, who wants to make protesters look like violent bums? This is a trick many hundreds of years old. The police, or people who are under pressure by police because of charges against them, dress up and break windows and start fires. They saunter through their dirty work with no hurry and no fear of arrest or even camera shyness. Somehow this is all clearly filmed by a "journalist" who then serves it up to social and corporate media. Commentators are in place and ready to be "appalled" and "shocked" by such behavior.
After the media blitz, the acts are forgotten and no one is arrested or charged.
In big events these police agents collect together and call themselves Blackbloc.