Pope Francis ushered the world's 1.2 billion Roman Catholics into Christmas on Wednesday, urging them to allow God to enter their lives to help combat darkness and corruption.

The 78-year-old Argentine pope led a solemn Christmas Eve Mass for thousands of people in St Peter's Basilica. It is the second Christmas season for the pope, who was elected last year as the first non-European pontiff in 1,300 years.

He has brought an air of simplicity to the Vatican, refusing many of the trappings of office, and has made plain his determination to reform the Vatican and bring the Church's hierarchy closer to rank-and-file Catholics.

In his homily, Francis, wearing white vestments, said Christmas is a time to remember that God's message of peace "is stronger than darkness and corruption"

"The question put to us simply by the infant's presence is, 'Do I allow God to love me?'" he said. "Do we have the courage to welcome with tenderness the difficulties and problems of those who are near to us ...?"

"How much the world needs tenderness today!" he said.

Hours before the service, the pope made a surprise telephone call to comfort Christian refugees in a camp in Ankawa, Iraq, who were about to celebrate their own Christmas Eve Mass.

"You are like Jesus on Christmas night. There was no room for him either, and he had to flee to Egypt later to save himself," Pope Francis told them in the call arranged by the Italian Catholic television station Sat2000.

The refugees fled Islamic State fighters who have persecuted Shi'ite Muslims, Christians and others in Syria and Iraq who do not share the group's ideologies.

The pope has several times condemned the "barbaric violence" of Islamic State fighters, most recently during his trip to Turkey last month.

On Thursday morning, the pope will deliver his traditional Christmas Day "Urbi et Orbi" (to the city and the world) blessing and message from the central balcony of St. Peter's Square to tens of thousands of people.

In Christmas greetings on Monday to the Vatican's top administrators, Pope Francis delivered a stinging critique of Vatican bureaucracy and outlined 15 illnesses plaguing the Curia, including "spiritual Alzheimer's".