Different theories of the origins of democracy propose that collective action by particular urban social groups spur democratization. But, recent large n studies on mass opposition movements and democratization notwithstanding, systematic and direct large n evidence on this ismissing. We further develop the argument that both industrial worker and urban middle class movements are conducive to democratization, albeit for slightly different reasons. We thereafter conduct the first large n empirical study of its kind on this issue. To this end, we collect new data on social composition of anti regime opposition movements, globally, for 1900 to 2012. Results clearly show that movements dominated by one of the two urban groups are associated with increased democratization prospects, both when compared to other movements and to situations without any organized mass opposition movement. We conduct various tests to assess sensitivity to, e.g., measurement strategy, geographic and temporal scope, model specification, and to accounting for the endogeneity of opposition campaigns. The relationship between industrial worker campaigns and democratization is particularly robust, but there is also evidence that middle class campaigns matter. Further tests suggest that these campaigns, as expected, relate more strongly to democratization in urban than in rural societies.