Pope Francis led the world's 1.2 billion Roman Catholics into Christmas on Thursday, urging those "intoxicated" by possessions and superficial appearances to return to the essential values of life.

Celebrating a Christmas eve Mass in St Peter's Basilica, Francis, whose nearly three-year-old papacy has been marked by calls for sobriety and compassion for the less fortunate, said Christmas was the time to "once more discover who we are".

He said everyone should allow the simplicity of the child Jesus, born into poverty in a manger despite his divinity, to infuse their spirit and inspire their lives.

"In a society so often intoxicated by consumerism and hedonism, wealth and extravagance, appearances and narcissism, this Child calls us to act soberly, in other words, in a way that is simple, balanced, consistent, capable of seeing and doing what is essential," he said in his homily.

The service for about 10,000 people in St Peter's Basilica started with a long chant in Latin, known as the Kalenda, the traditional proclamation of the birth of Jesus.

The great bells of St. Peter's then rang out and the pope, dressed in white vestments, kissed a statue of the infant Jesus to start the solemn Mass.

Security was tighter than normal for Christmas, with many police carrying out spot checks in the Vatican area. Everyone who entered the basilica, the largest church in Christendom, went through metal detectors.

The 79-year-old Argentine pope encapsulated in his homily some of the key themes of his papacy: mercy, compassion, empathy and justice.

"In a world which all too often is merciless to the sinner and lenient to the sin, we need to cultivate a strong sense of justice, to discern and to do God's will," he said.

Francis, who said earlier this week he had a slight flu, seemed tired and spoke with a slightly hoarse voice at times.

While not changing fundamental Church doctrine, Francis has been calling for a more merciful and less judgmental Church, one that is more compassionate toward groups such as gay people and the divorced who have civilly remarried.

Conservatives have criticized some of his statements, such as the now-famous "Who am I to judge" comment about homosexuals who were seeking God and had good will. The conservatives say statements like these only sow confusion among the faithful.

The pope said child Jesus was calling on everyone to rethink the way they treat others.

"Amid a culture of indifference which not infrequently turns ruthless, our style of life should instead be devout, filled with empathy, compassion and mercy, drawn daily from the wellspring of prayer," he said.

On Christmas day, Francis will deliver the traditional "Urbi et Orbi" (to the city and the world) message from the central balcony of St. Peter's Square, from where he first appeared to the world after his election on March 13, 2013.