You don't say which country or police force but assuming a general question, According to Wikipedia Wikipedia:History_of_the_Metropolitan_Police_Service the Met one of the earliest 'modern' police forces in the UK (and possibly the world) was formed in 1829, although the article notes that an organised investigative force existed as early as 1753 ( see Wikipedia:Bow Street Runners) Prior to this the UK had 'watchmen' and 'constables' who kept law and order, but were not an organised 'police' as such.