Pope Francis was due Tuesday to lead his first Christmas celebrations since his election this year as leader of the world's 1.2 billion Catholics, dpa reported.
Francis was due to celebrate Mass in Saint Peter's Basilica at 9:30 pm (20:30 GMT).
Thousands of pilgrims were expected to attend the service, which is one of the grandest in Catholic liturgy and is to be presided over by a pontiff who has tended to shun pomp and protocol in the first nine months of his papacy.
Ceremonies started in the afternoon with the unveiling of a life-sized nativity scene in the middle of Saint Peter's Square next to a 25-metre Christmas tree from Bavaria, Germany.
The manger scene, built by Naples craftsmen, was opened for public viewing as the Vatican band played the traditional Neapolitan song O Sole Mio.
On Monday, Francis prepared for the festivities by paying a visit to his predecessor, Pope Emeritus Benedict XVI, who in February became the first pope in 600 years to resign.
He also urged the faithful to shun the material aspects of Christmas to concentrate on its spiritual meaning.
"Is there a place for the Lord or only for parties, for shopping, for revelry?" he preached in a sermon reported by Vatican Radio. He insisted that the souls of Catholics should be open, rather than "closed with a 'do not disturb' sign."
He made a similar point in a Twitter message. "Christmas celebrations are often full of sound. It would be good for us to make room for silence, to hear the voice of love," Francis wrote.
At noon on Christmas Day, the pope is to deliver traditional blessings and the multilingual Urbi et Orbi (to the city and to the world) message, speaking from the central balcony of the Vatican basilica.