How did the term "X's finest" come to mean the police force of a city X?

I have often come across terms like London's finest, New York's finest, etc., intended to mean the police forces of the respective cities. I think in the case of Scotland Yard, the term even has some official currency. How did this usage originate? Was it sarcastic or propaganda?

It's hard to believe that the finest people of a city are its policemen, especially given how often popular tales involve thieves and outlaws as protagonists.

In the case of "New York's Finest," Barry [Popik] has traced the term back to the 1870s, where it apparently first emerged in the form "the finest police force in the world," a phrase associated with George Matsell (police chief at the time), and possibly modeled on Civil War Major General Joseph Hooker's estimation of his troops as "the finest army on the planet." The general idea of New York's police being "the finest" had been asserted in print for several years by that point, so it wasn't surprising that eventually the tribute was codified in the phrase "New York's Finest."

http://www.word-detective.com/0806C.html

I've never heard the police in London referred to as "London's finest", and a search for that phrase results in must-see landmarks, expensive hotels, hairdressers, dance troupes and man-and-van services. But adding the word police shows the Metropolitan Police do (or have) used the phrase:

A career with London's finest

http://content.met.police.uk/News/A-career-with-Londons-finest/1260267625884/1257246745756

I don't think it's an official term in London, unlike New York and some other American cities.

In NYC, the police are "NY's Finest", the firemen are "NY's Bravest" and the sanitation workers are "NY's Strongest". ("Everybody wants to get into the act" - Jimmy Durante)