Cops are patrolling the city, looking for delinquents to arrest with extreme prejudice. They come in different flavors, from the Tonga equipped melee character to the deadly shotgun wearing cop. In general, cops have above average combat statistics and skills. Be also aware that, like any other citizen, cops have off duty periods when they won't be actively pursuing enemies except when directly attacked.
They can be found all over the city, however they are more frequent in City Center and Industrial districts. They also patrol around shops, factories and parks frequently. Resident areas, like the Suburbs are much less likely to be swarming with cops.