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Anyone who is involved in any sort of protest should video and/or photograph as much as possible with their cell phone (assming it is capable, which a lot are) and immediately upload the images/videos to wherever. Such documentation is what caught the police agents provoacateur at a protest in Canada a couple of years ago. The police, faced with pictures of the agents provocatuer wearing the exact same boots as the police do, eventually had to admit they were involved in such a scurrilous enterprise.
Document, document, document.. The more camera angles you have the more accurately the events can be reconstructed in case that become necessary.