If I understood you correctly, you just name them. The default is the concept, and most of the time people do not feel the need to separate some specific police force, because it's apparent from the context. Some specific police force aren't likely to expand their operations in the city.
On the other hand, I think I may also be failing to explain this correctly because we are already at the limits of my English :)
On the other hand, I think I may also be failing to explain this correctly because we are already at the limits of my English :)