Many of the tens of thousands attending were Catholics, but tourists also came to be part of the historic occasion.
Joe from the US state of Missouri said: "We're on vacation, but it's great timing. We're here to see the Pope's inaugural Mass. It's very special. I'm glad we came early."
He said he was "extra proud" to see the first Pope from the United States. "That was a surprise. He's gonna be a wonderful Pope. I am not Catholic, but I grew up Catholic, but this is just inspiring no matter what denomination of Christian you are."
Also in the crowds was Pia, from Chile, a professor of philosophy at the Pontifical University of the Holy Cross in Rome. She told the BBC she felt there was "a new hope in the church".
Pia said that among the Pope's first words when he was elected was "Let the peace be with you", the words of Jesus. And then he said "don't be afraid".
"He knows what the world and the church needs. A church that is preaching hope, preaching peace. I think many people are waiting for that," she said.
The Pope's official inauguration followed the Mass, with a pallium garment – a white woolen band – placed on the pope's shoulders, and fixed in place with three pins to represent the nails on the cross.
Cardinal Luis Antonio Tagle of the Philippines then placed on the pope's finger the Ring of the Fisherman, a symbol of the papacy which bears an image of St Peter.
Pope Leo then took the book of the gospels to bless the people.
