The explosive growth of so-called marginal settlements in Latin America's cities has received considerable academic attention in past decades. The inability of the state to adequately provide affordable housing for a rapidly growing urban population has meant that these neighbourhoods relied on community mobilization to achieve common objectives of basic infrastructure and legal recognition. With increasing consolidation and less need for coordinated action, many local organizations lost their rationale. Fear of crime and violence, a major concern in Latin American metropolises, is inscribed on the urban landscape by a growing number of gated and fortified residential enclaves for the better-off. Marginal neighbourhoods, however, experience a ‘security gap’ as they receive insufficient police protection and lack the financial resources to employ private guards. In many cases, the real or perceived insecurities of inner-city life have prompted organized bottom-up, mostly informal and sometimes vigilante, responses. Drawing on research on the rise of informal security-related interventions in the neighbourhoods of Lima Metropolitana, Peru, this paper explores the rationales underlying the different approaches and adoptions, including the involvement of the main actors, community associations and other local interest groups.