That tension—between the police as a public good investigating and solving particular crimes and the police as a tool of suppressing dissent—has been with us since at least 1829 (the foundation of the London Metropolitan Police). And I suppose I do think this combination is new, or new-ish.
More specifically, while armed thugs have been with humanity forever, I'm tempted to argue that the *armed thug who has the social reputation and seeming benevolence of a Noble Crime-Solver" is new, or more precisely one of the major social innovations of the nineteenth century.
One potentially interesting way to get into this: it'd be cool to figure out the first children's book that presented "policeman" as a desirable career one might want to have and not a glorified bouncer.
New or not, it's definitely modern.