SPQR

SPQR are the initials of a Latin phrase "Senātus Populusque Rōmānus". It means "The Roman Senate and People" or "The Senate and People of Rome". 
It refers to the government of the ancient Roman Republic. It is still used as an official emblem of the modern-day municipality of Rome. 
SPQR appears on Roman coins, at the end of documents made public by inscription in stone or metal, in dedications of monuments and public works, and on the flag (vexilloid) of the Roman legions.
The phrase appears in Roman political, legal and historical literature, such as the speeches of Cicero and "Ab Urbe Condita Libri" ("Books from the Founding of the City") of Livy.
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"