While nobody was injured in the explosion, a car was damaged and many workers at the Poste Italiane office near Aventine hill were sent into a panic.
Witnesses said they heard two blasts, one of which was stronger than the other.
Investigators said the homemade device was equipped with a timer and comprised two bottles of flammable liquid inside a plastic container.
Prosecutors have opened an investigation into the explosion, and are still unsure of the intended target.
Investigators suspect that Italian anarchists may have planted the bomb, as such groups have been responsible for several similar arson attacks on Italy’s postal service in the past.
A similar explosive device that failed to blow up was found nearby a Turin post office in November.
Investigators are studying video footage of the car park where the bomb was planted in an attempt to identify those responsible.
