The thesis here achieved is the result of reflections arising still in the Master of Social Work the research undertaken at that time revealed historical documents that allowed to infer that must be credited police institutions the same dramatic weight and importance of charity and philanthropy in the formation of the social assistance field in the city of Rio de Janeiro. To demonstrate the relevance or not of this inquiry we developed historical research from the reports of the Police Chiefs and Justice Ministers which allowed us to affirm that the social assistance was a policy of State for coping with urban poverty where the police institutions appear as one of the main executors and idealizers of these social practices. Our study allows us to affirm that the social assistance was an important factor in the consolidation of the nation-state and modernization and the civilization of the city of Rio de Janeiro through the municipality administered by police. We identified the logics present in the social practices and knowledge constructed by police who have become hegemonic for coping with the problems of urban poverty through the social assistance. Knowledge and practices that have influenced in the formulation of laws public administration and construction of a collective imagination about the representations of poverty in the city of Rio de Janeiro. In this sense we will discuss how the social assistance can be considered a legitimating mechanism of the State in the formation of the nation through police institutions.