Group From 29/9/1829
Founded in 1829 by Robert Peel under the Metropolitan Police Act 1829 and on 26 September of that year, over 1,000 men were sworn in in the grounds of the Foundling Hospital. (From Sarah Wise's book, The Italian Boy).
The next year local divisions, one for each borough and each with its own station, were established. These were known by letters of the alphabet. e.g. M division was for Southwark.
The Met did not police the City, so, predictably, territorial disputes materialised. The old-style force policing the City moving vagrants over the border and the Met doing the same in reverse. Temple Bar was a location where this activity became a spectator sport (from Sarah Wise's book). Sad and laughable as this now seems, in essence, the practice has not gone away: the buskers at King's Cross knowing the precise line where the BT police patch meets that of the Met and what the differing busker policies are; each Council being aware then whenever they launch a campaign to clean up drug-dealing, it causes increased drug-related activity in neighbouring boroughs.
Wikipedia states the Met is "the first modern and professional police force in the world" though the establishment of the Marine Police preceded the land-based force by over 30 years, and Commissioners of Police for Scotland were appointed in 1714. It's all in the definition.