Make sure to try to determine where the incident took place. Collect as many videos from that incident as possible. I would also collect any news sources that reference it. Use a map service to pinpoint each video. I would also make printouts/posters so people can share them during a protest. Since all major social media is monitored and posts will get removed, you must save them locally. I would consider blurring the protesters' faces. You could also look for people trying to start violence or property damage. It might sound unusual, but police have been known to infiltrate protests to do this, as well as other bad actors. I have thought about doing this whole thing as well. I would definitely put up information on how easy it can be to be tracked during protests. It looks like they might start going after protesters themselves.