PREFECT (Prefectus urbi) was originally warden of the city of Rome, and received the name of Custos Urbis. The title of prefect does not seem to have been used till after the time of the Decemviri. On the absence of the king from Rome, the custos urbis acted as his representative; and exercised all the powers of the consuls if they were absent. Persons of consular rank were alone eligible for this office; and, down to the time of the Decemvirate every prefect had previously been consul.
On the institution of the office of prator urbanus, the wardenship of the city became a mere name. In the reign of Augustus, at the suggestion of Mæcenas, an office bearing this designation was instituted, but widely different from the original. This new praefectus urbi was a permanent magistrate, with the superintendence of butchers, bankers, theatres, &c.; and to enable him the better to exercise his authority, he had distributed throughout the city a number of milites stationarii, a sort of modern police. Gradually his office extended, until he in turn swallowed up the office of prator urbanus. On Constantinople becoming the second capital of the empire, it had its praefectus urbi; who were the direct representatives of the emperor. All the officers of the administration of the city—all corporations and public institutions were under their supervision. They received the petitions of the people addressed to the emperor; and when the capital deigned to present the emperor with an expression of their loyalty, it was through the prefect they had to approach him. (Smith's Dictionary of Greek and Roman Antiquities.)