<P> The Roman Empire ( Latin : Imperium Rōmānum ; Classical Latin : ( ɪmˈpɛ. ri. ũː roːˈmaː. nũː ) Koine and Medieval Greek : Βασιλεία τῶν Ῥωμαίων , tr . Basileia tōn Rhōmaiōn ) was the post-Roman Republic period of the ancient Roman civilization , characterized by government headed by emperors and large territorial holdings around the Mediterranean Sea in Europe , Africa and Asia . The city of Rome was the largest city in the world c. 100 BC -- c . AD 400 , with Constantinople ( New Rome ) becoming the largest around AD 500 , and the Empire 's populace grew to an estimated 50 to 90 million inhabitants ( roughly 20 % of the world 's population at the time ) . The 500 - year - old republic which preceded it was severely destabilized in a series of civil wars and political conflict , during which Julius Caesar was appointed as perpetual dictator and then assassinated in 44 BC . Civil wars and executions continued , culminating in the victory of Octavian , Caesar 's adopted son , over Mark Antony and Cleopatra at the Battle of Actium in 31 BC and the annexation of Egypt . Octavian 's power was then unassailable and in 27 BC the Roman Senate formally granted him overarching power and the new title Augustus , effectively marking the end of the Roman Republic . </P>
what was the population of the roman empire at its height
50 to 90 million inhabitants