If it is guaranteed to be a violent riot, a mask does little to quell the situation. Granted, there are kitties who would act up when they think they are anonymous (the internet is a perfect example). However, if there are no consequences (police told to stand down), then they will be emboldened. ***** them up and the kitties will be less apt to contribute. At that point, it will just be the paid protestors, which you are not going to stop anyway.