The fall of Suharto’s authoritarian regime and the subsequent dissolution of vertical political patronage led to an upsurge of mass mobilization based on religion and/or ethnicity. In Jakarta, newly emerged vigilante groups that initially sought to represent small-scale neighborhood communities rapidly grew in size by receiving endorsements from local political authorities as well as by gaining extensive popular support. Despite their persistent association with violence and illicitness in popular discourse, some of those vigilante groups quickly increased their membership to hundreds of thousands. Highlighting the activities of the Forum Betawi Rempug (FBR), one of the biggest of these groups, this paper explains the causes, processes, and consequences of its expansion. The nature of the Betawi ethnic identity that has been constructed over decades, as well as an alternative mode of populist discourse that became prevalent in Jakarta during the last couple of decades, were the key background conditions through which such groups expanded in both size and geographic reach. These conditions also led to a loosely disciplined and highly autonomous organizational structure. An explanation of this process calls for a radical revision of the conventional model of ethnic mobilization that takes for granted disciplined organization and hierarchical control. In contemporary Jakarta, successful mass mobilization is not the sheer result of people’s response to populist calls. Attention must be paid to the logic of the mobilized in order to explain why vigilante organizations have been able to gain popular support despite their notorious reputation. This paper investigates the perspectives of the mobilized by focusing on neighborhood-level activities of the FBR. In so doing, it exemplifies how some residents perceive the FBR as a provider of potential socioeconomic resources for the enhancement of their life environment.