“Two days later it was Christmas. St. Peter’s Basilica was full of people wearing colourful clothing and shiny ornaments. Charlemagne, king of the Franks, entered, wearing sandals and dressed in the Roman style with a long tunic and a cloak. He proceeded to the altar, where he knelt down and prayed for a long time in silence. Leo III, the pope, walked towards him and placed a golden crown on his head. All the people cheered and shouted three times: ‘To the Emperor Charles Augustus, crowned by God, noble and peaceful emperor of the Roman, life and victory!’. The noise was very loud. The pope knelt before the emperor and kissed his cloak. Charlemagne became the emperor of the West. He was the first since the fall of the Roman Empire in 476”.
F. Lebrun. The days of Charlemagne
Here you have as well a documentary about Charlemagne, in case you want to study further about him: