The legal regulation of social protest takes many forms: injunctions, municipal by-laws relating to breaches of the peace and public order, traffic regulations, criminal offences, etc. The multiplication of legal norms, combined with a harshening of police practices during demonstrations, led to an increased criminalization of protesters in Canada in the last twenty years. Focusing on this phenomenon, the author argues that in spite of the increasing use of regulatory offences, criminal offences continue to be an important tool in the hands of authorities. This is particularly true in the case of the offence of being a member of an unlawful assembly. Yet the current definition of this age-old notion reveals a number of tensions with central tenets of modern criminal law, namely the principle of legality, the idea of an individual fault, and its post hoc orientation.