Who crowned Charlemagne?

Pope Leo III crowned Charlemagne as emperor on Christmas Day, 800, in St. Peter's Basilica, Rome.

He was given the title "Imperator Romanorum", or "Emperor of the Romans". Leo had been supported by Charlemagne in a power struggle in Rome, and was both honoring Charlemagne and simultaneously discrediting the legitimacy of Empress Irene of Constantinople by declaring him Emperor. Charlemagne's biographer claimed Charlemagne did not know Leo was going to bestow this title on him, but many historians are suspicious of this account.