THE CORONATION OF CHARLEMAGNE: Since the title of emperor had become extinct among the Greeks and a woman (Empress Irene) claimed the imperial authority, it seemed to Pope Leo and to all the holy fathers who were present at the council and to the rest of the Christian people that Charles, king of the Franks, ought to be named emperor, for he held Rome itself where the Caesars were always accustomed to reside and also other cities in Italy, Gaul and Germany. Since almighty God had put all these places in his power it seemed fitting to them that, with the help of God, and in accordance with the request of all the Christian people, he should hold this title. King Charles did not wish to refuse their petition, and, humbly submitting himself to God and to the petition of all the Christian priests and people, he accepted the title of emperor on the day of the Nativity of our Lord Jesus Christ and was consecrated by Pope Leo.
Vita Leonis III (795-816)
On the day of the Nativity of our Lord Jesus Christ all [who had been present at the council] came together again in the same basilica of blessed Peter the apostle. And then the venerable and holy pontiff, with his own hands, crowned [Charles] with a most precious crown. Then all the faithful Romans, seeing how he loved the holy Roman church and its vicar and how he defended them, cried out with one voice by the will of God and of St. Peter, the key-bearer of the kingdom of heaven:
To Charles, most pious Augustus, crowned by God, great and peace-loving emperor, life and victory.
This was said three times before the sacred tomb of blessed Peter the apostle, with the invocation of many saints, and he was instituted by all as emperor of the Romans. Thereupon, on that same day of the nativity of our Lord Jesus Christ, the most holy bishop and pontiff anointed his most excellent son Charles as king with holy oil.