Urban Security, also known as UrbSec (or U-Sec in some sources), is the Imperium's regulary police force, responsible for maintaining law and order on a daily basis.
In the early years of Sidh colonization, there was little need for a dedicated police force. The first Sidh colonies operated under what was effectively martial law, every citizen being essentially regarded as military personnel and subject to military-style regulations. Crime of any kind was rare, even rebels and misfits recognizing the need to cooperate and fall in line in order to survive, and when crimes did happen, they were usually dealt with swiftly and mercilessly by the colonists themselves.
With the gradual expansion of the Imperium and the partial restratification of society into civil and military spheres, the authority and peer supervision relaxed and crime again appeared on the rise, colonial militias responding with vigilantism and sometimes excessive force, necessitating the formation of a dedicated police force. Consequently, the Emperor issued the Urban Security Charter of 2082, a set of standards and regulations for this emergent police force.
Due to the sheer technical difficulty of running a centralized bureaucracy on an interstellar scale, especially back in the day when interstellar communications were still in their infancy, the new model of police force was envisioned as decentralized, each planet raising it's own force in accord with the unified standards set by the Charter.
Urban Security isn't a civil police force, but rather a heavily-armed gendarmerie that also serves as planetary garrison along with the Auxilia citizen milita units. Consequently, UrbSec troops are much more heavily equipped than an ordinary police force, down to having their own heavy armor and even attack aircraft and artillery as one of their duties would be to serve as first-responders in the event of a planetary invasion, holding on their own until the military can muster a proper response. Their military-issue equipment also serves to provide for an intimidating presence deemed necessary to deter crime - while Sidh-on-Sidh crime is generally rare, the prevalence of augmentation, combat training and a lack of cultural taboos against the use of violence means that the results tend to be spectacularly-violent when it does happen.
Unlike Domestic Security, which maintains an Imperium-wide mandate and a centralized command structure, each Urban Security department only has local authority. Cases that transcend the boundaries of their homeworld are forwarded to Domestic Security to pursue. Domestic Security can likewise commandeer local Urban Security forces to deal with a problem on their behalf, especially seeing how Domestic Security typically lacks the heavy equipment possessed by UrbSec. For these reasons, UrbSec cannot really be described as a single organization, but rather a collection of identical independent organizations.
Urban Security, like other Imperial government enforcers, has a rather fearsome reputation, perhaps undeservedly so. While UrbSec patrols no doubt project an intimidating presence with their burly men clad in massive military-issue powered armor, wielding baseball-bat-sized stun sticks and autocannon-caliber sidearms, law-abiding citizens have nothing to fear. Lawbreakers, however, can likewise expect little mercy from these uncompromising enforcers of Imperial law. The notoriety of Urban Security largely derives from the misadventures of non-Sidhae unfamiliar with the UrbSec's standard policy of no third warnings, Sidh citizens usually knowing better than to resist, flee or disobey when confronted by an UrbSec official. The standard protocol rigidly observed by UrbSec is to issue two warnings only, failure to comply after the second warning being deemed resistence to lawul demands and dealt with swiftly and harshly. UrbSec troops likewise have no qualms or regulations against dispensing summary punishments for minor offenses not deemed worthy of a justicar's attention, such as petty theft or public drunkenness, the offender usually being let off after a good beating.
That said, UrbSec troops, like most other representatives of Imperial authorities, are held to high standards of conduct for the broad authority they are granted, minor offenses that would only yield a fine to a common citizen being crimes punishable with imprisonment to UrbSec troops.
While popular with the Sidh populace as opposed to Domestic Security, UrbSec is much resented by the human residents of the Imperium, seeing how one of their duties is to keep the ghettos pacified. This isn't helped by the fact that it's a common practice on many worlds to use ghetto duty as disciplinary punishment, ghetto patrols becoming the dumping grounds for misfits and humans consequently regarding UrbSec as corrupt and arbitrary jackbooted thugs. This is obviously not always the case, idealistic UrbSec troops occasionally volunteering for ghetto duty with a sincere desire to bring law and order to the lawless and improve the safety of the ghetto denizens, only for their efforts to be frustrated by public hostility and the arbitrary and dishonest practices of their own unscrupulous comrades.
Urban Security is usually subject to the planetary governors of their respective homeworlds. In times of war, they fall under military jurisdiction and command.