This paper models a dynamic sequence of protest activities as an informational cascade. Such a synthesis of participation and signaling games motivates why large numbers of people may have incentives to engage in costly collective action. The haphazard dynamics of their protest are endogenously driven by the revelation of information through changes in the size of the protest movement over time. In some cases, a small number of political actions may have a large impact on public opinion; in other cases, a huge turnout may be followed by the sudden collapse of the protest movement. The political impact of mass political action is determined by the difference between actual and expected turnout, rather than sheer numbers. The participation of individuals with moderate preferences is crucial for the success of the protest movement, while extremist turnout is discounted.I provide some empirical support for this theory with a study of the Leipziger Montagsdemonstrationen (Monday demonstrates in Leipzig) that played a crucial role in the East German revolution of 1989/90.