Pope Francis opened the "Holy Door" of St Peter's Basilica on Christmas Eve on Tuesday, launching the Jubilee year of Catholic celebrations set to draw more than 30 million pilgrims to Rome.

The 88-year-old pontiff, who has recently been suffering from a cold, was pushed in a wheelchair up to the huge, ornate bronze door and knocked on it, before the doors opened.

In a ceremony watched on screens by thousands of faithful outside in St Peter's Square, the Argentine pontiff went through the door followed by a procession, as the bells of the Vatican basilica rang out.

Over the next 12 months, Catholic pilgrims will pass through the door, which is normally bricked up, by tradition benefiting from a "plenary indulgence", a type of forgiveness for their sins.

Pope Francis then presided over the Christmas Eve mass in St Peter's, where he turned once again to the victims of war.

"We think of wars, of machine-gunned children, of bombs on schools and hospitals," he said in his homily.

The pope had drawn an angry response from Israel at the weekend for condemning the "cruelty" of Israel's strikes in Gaza that killed children.

Some 700 security officers are being deployed around the Vatican and Rome for the Jubilee celebrations, with measures further tightened following Friday's car-ramming attack on a Christmas market in Germany.

Much of Rome has also been given a facelift in preparation, with monuments such as the Trevi Fountain and the Ponte Sant'Angelo cleaned up and roads redesigned to improve the flow of traffic.

Many residents have questioned how the Eternal City, where key sites are already overcrowded and public transport is unreliable, will cope with millions more visitors next year.

Key Jubilee projects were only finished in the last few days after months of work that turned much of the city into a building site.

Inaugurating a new road tunnel at Piazza Pia next to the Vatican on Monday, Prime Minister Giorgia Meloni said it had taken a "little civil miracle" to get the project finished in time.

Over the course of the next few days, Holy Doors will be opened in Rome's three major basilicas and in Catholic churches around the world. (AFP)