This paper presents an agent-based model to understand the emergence of aggressive mob during a protest. We study to what extent (1) protester behavior can lead to violence and (2) protest policing doctrine. The multi-agent architecture relies on the Belief Desire Intention paradigm, enriched with emotions, norms and personality. Indeed, to reproduce phenomena such as tension-building, the protester model includes emotional components that are added to Epstein's model of civil violence. In Epstein's model, aggressive behavior is triggered as a tradeoff between grievance and perceived risk of arrest. Our model is implemented on the open-source GAMA platform and applied to the study of two factors observed in real-life situations: policeprotester ratio and protest configuration (police blockade of escape routes, also called kettling, or police moving in the crowd). Simulations show that the key parameters are not necessarily the police-protester ratio but rather the protest configuration.