Model for urban US police departments was London force established in 1829 as a response, not to criminality per se, but to growing 'dangerous class' of destitute in industrializing England. From their beginning, modern police have been conscious creations of those seeking to protect existing social structure, hence, they have been officially sanctioned enforcers of prevailing arrangements. Regular presence of police, particularly among working class, allowed them to premeate society in ways that traditional constables could not. They represented extension of central political authority throughout daily life.
The role of the police, as we relate to them, is contradictory and influenced by circumstances (So yeah, when I was robbed in my house at gun point years ago I called the cops, upon their, took a look around and decided that what had happend was a hippie drug ripoff.) Police penetration of poor neighborhoods, for example, is so complete that they cause increased tension and anxiety while disrupting community life. Individual officers may - or may not - 'serve and protect' - some get harrassed, some are overworked, some are authoritarian, some are racist. But collectively, police are guardians of prevailing social relations who often see world as divided between "us" and "them." Thus, police ethic justifies any action intended to maintain dominant conception of order.
As Marx pointed out, Paris Commune stripped police of "political attributes" in its becoming 'responsible and at all time revocable agent of the Commune.' And with, no doubt, some hyperbole he pointed out that there were "no more corpses at the Morgue, no nocturnal burglaries, scarcely any robberies" after the "merely repressive organs of the old government" had been "amputated." Michael Hoover