Pope Francis has hailed God's guiding power and urged the faithful to be selfless as he conducted Christmas celebrations for the first time since his election this year as leader of the world's 1.2 billion Catholics.
Francis led a procession in front of thousands of faithful and senior clergymen, who packed Saint Peter's Basilica for a Christmas Eve Mass that started at 9.30pm local time and lasted less than two hours.
"God is light, and in him there is no darkness at all," the pontiff said in a short homily, quoting from the Gospel of John.
In a change from practice, he personally kissed and placed a baby Jesus in a manger at the start of the solemn service.
At the end, Francis placed the statue in a nativity scene, then kissed a group of children on their foreheads.
"If we love God and our brothers and sisters, we walk in the light; but if our heart is closed, if we are dominated by pride, deceit, self-seeking, then darkness falls within us and around us," he said.
The Argentine pontiff, who has shunned pomp and protocol in the first nine months of his papacy, presided over one of the most elaborate ceremonies in Catholic liturgy, set in one of the world's grandest buildings.
The service is physically taxing for reigning popes.
In recent years it was moved earlier into the evening to allow them to have a few more hours' sleep before resuming their duties on Christmas Day.
On a regular day, the 77-year-old Francis wakes up before 5am and goes to bed before 10pm.
On Wednesday, he was due to deliver multilingual messages and the Urbi et Orbi (to the city and to the world) blessings at noon, from the central balcony of Saint Peter's.
Christmas celebrations at the Vatican started on Tuesday afternoon, with the unveiling of a life-sized nativity scene in the middle of Saint Peter's Square, next to a Christmas tree from Bavaria, Germany.
Built by Naples craftsmen, the crib was opened for public viewing as the Vatican band played the traditional Neapolitan song `O Sole Mio.
In another break from custom, the inauguration was not followed by the pope appearing from the window of his study to light the traditional "peace" candle.
Instead, the candle was placed near the nativity and lit by Italian Cardinal Angelo Comastri.
In the run up to Christmas, Francis paid a visit to his predecessor Benedict XVI, who in February became the first pope in 600 years to resign.
Francis urged the faithful to shun the material aspects of the feast to concentrate on its spiritual meaning.
"Is there a place for the Lord or only for parties, for shopping, for revelry?" he asked on Monday.
He insisted that the souls of Catholics should be open, rather than "closed with a 'do not disturb' sign"
