A Police force is a constituted body of persons empowered by the state to enforce the law, protect property, and limit civil disorder. Their powers include the legitimized use of force. The term is most commonly associated with police services of a sovereign state that are authorized to exercise the police power of that state within a defined legal or territorial area of responsibility. Police forces are often defined as being separate from military or other organizations involved in the defense of the state against foreign aggressors; however, gendarmerie are military units charged with civil policing. The police force is usually a public sector service, meaning they are paid through taxes.
Starfleet employed its own police force to deal with personnel that broke the law, usually drawn from security. Arthur Porter was a police agent for Starfleet's internal police prior to joining False Vacuum Squadron. (Star Trek: False Vacuum: "Arthur Porter and the Corpse of Azkapanama")
The Terran Empire had police forces separate from Starfleet and Military Assault Command Operations. One of these forces saved a Tellarite from an anti-alien lynch mob in London following the Xindi attack on Earth.