Violent protest strategies have often been considered as less effective than non-violent action for achieving the goals of movements. The main proposed mechanism is that non-violent action lowers the participation barriers and costs, and therefore is able to increase participation. In this paper we explore a complementary mechanism: the impact of violent tactics on popular support and perception of protest movements. Public support is a powerful resource for social movements, even beyond direct involvement, so it is key to understand the consequences of tactic choices for how the movements are perceived by the public. Often, the causal effect of these choices is difficult to estimate. In this paper we exploit a natural experiment in the context of the 15M (indignados) movement in Barcelona: a series of riots that took place during the fieldwork of a face-to-face survey in Barcelona. Our results show that, on average, rioting reduced support for the movement in 7 percentage points. Moreover, we find that this effect is highest for the ‘weak supporters’ of the movement (around 16 points) and null for its core supporters. Results are robust to different specifications. These findings have potentially important implications for protest movements concerned with broadening their support base.