Their primary role was to police Rome and to counteract the roaming mobs and gangs that so often haunted its streets during the Republic. The urban cohorts thus acted as a heavy duty police force, capable of riot control duties, while their contemporaries, the Vigiles, had the day-to-day role of policing the streets and protecting against fires. As a trained paramilitary organization, the urban cohorts could, on rare occasions, take to the field of battle if necessary. This role, however, was only called upon in dire situations. Augustus established a city police force at Rome consisting of three urban cohorts (cohortes urbanae) under a newly appointed prefect of the city. By this time the gangs of Titus Annius Milo, Publius Clodius, etc. which had been used by politicians during the Republic had been eliminated, mostly due to the efforts of Pompeius Magnus and, with the founding of the Principate, had become moot since power no longer resided in the Roman Senate and elected officials. Unlike the Vigiles, who mostly operated at night as firewatch and watchmen, members of the urban cohorts were considered legionaries, though with higher pay than the regular legions—if not quite as much as the Praetorian Guards—and tended to receive slightly higher donatives though, again, not as much as the Praetorians.
Originally the cohortes urbanae were divided into three cohorts, each cohort being commanded by one tribune and six centurions. In the time of the Flavians this was increased to four cohorts. Each cohort contained around five hundred men. Only free citizens were eligible to serve in their ranks. As with the Praetorians, the men of the urban cohorts were predominantly of Italian stock. Urban cohorts, (known as city cohorts in non-Roman cities) were later created in both the Roman North African city of Carthage and the city of Lugdunum in Roman Gaul (modern Lyon).