The date of first use of SPQR is not known. It first appears in inscriptions of the later Republic, from about 80 BC onwards. It last appears on coins of Constantine the Great (ruled AD 312-337), the first Christian Roman emperor.
The signature continued in use under the Roman Empire. The emperors were considered the representatives of the people, although the decrees of the Senate were dictated by the wishes of the emperor.
Similar language in more modern political and social revolutions no doubt comes from this usage. During the Fascist regime of Benito Mussolini, SPQR was emblazoned on many public buildings in an attempt to promote his dictatorship as a "New Roman Empire"