The Philippines has a national, centrally led police force, and I wouldn't exactly say its mandate is enforcing tyranny, though it is certainly corrupt. At one time the police were under the control of municipal, provincial, and city governments, and city governments, but they quickly became in many cases little more than government-paid thugs working for mayors and governors, with a definite mandate to enforce tyranny. That still happens, but at least the central authority has the ability to shuffle the deck and transfer officers that get too cozy with local authority... even if that ability isn't always used, or is used too late. Very much imperfect, but better than it was in the days of local control.

How that would work in Afghanistan I don't know... probably not very well, like everything else. There is no system that will not be corrupted if the people running it are corrupt. I'd only point out that a central police force isn't necessarily or at all times a worse or more tyrannical alternative. I guess that would depend on where the loci of tyranny - especially that tyranny that affects the common people - really are. Local governments can be every bit as tyrannical, within their bailiwicks, as central ones.