Pilgrims attend a papal mass in St Peter's square for the start of the Jubilee Year of Mercy, on December 8, 2015 in Vatican
Vatican City (AFP) - Tens of thousands of Catholic pilgrims assembled in Rome Tuesday to watch Pope Francis open a "Holy Door" in the walls of St Peter's basilica at the start of an extraordinary Jubilee year.
After a mass on St Peter's square, the leader of the world's 1.2 billion Catholics was to pronounce the words "Aperite mihi Porta Iustitiae" -- Latin for "open to me the gates of justice" -- and the door, which is normally bricked up, opened.
The first pilgrims had been in the square since before dawn in search of a prime spot to watch the latest enactment of a 700-year-old tradition laden with religious symbolism.
An estimated 50,000 people, including hundreds of cardinals, bishops and members of religious orders, attended an event subject to unprecedented security measures in the wake of recent terrorist attacks around the world.
As Francis initiated the mass, many of the pilgrims had tears running down their cheeks, others listened in silent contemplation or private prayer.
Images of the ceremony were beamed live around the world.
In Catholic tradition, the opening of "Holy Doors" in Rome symbolises an invitation from the Church to believers to enter into a renewed relationship with God.
Francis's predecessor Benedict XVI, who has lived in seclusion within the Vatican since retiring in 2013 because of failing health, has accepted an invitation to attend Tuesday's ceremony.
It will be a rare public outing for the German ex-pontiff, now aged 88 and said to be extremely frail. His last one was for the canonisation of former popes John Paul II and John XXIII in April 2014.
Tuesday evening will see images by some of the world's greatest environmental photographers projected onto the facade of Saint Peter's in an initiative linked to the ongoing global climate conference in Paris.
- Mercy, mercy me -
The Jubilee, which runs until November 20, 2016, was called by the pontiff with the express goal of allowing the Church to "make more evident its mission to be a witness of mercy".
The pontiff has made the idea of mercy the dominant theme of his papacy, insisting that it is not simply an abstract concept.
In a surprise move, he announced in September that for the duration of the Jubilee, priests would be given special dispensation to absolve women who have had abortions.
Some 800 priests around the world are to be designated "missionaries of mercy" tasked with encouraging higher levels of confession amongst believers. Those involved have been selected for their ability to preach well, understand human frailty and ensure the confessional is not experienced "like a torture chamber" as Francis himself put it.
Behind all this lies Francis's mission to make the Church seem less judgemental and more understanding, at times in the teeth of fierce resistance from traditionalists opposed to any relaxation of teaching on hot-button subjects such as homosexuality, divorce and unmarried cohabitation.
Traditionally, Catholics were expected to make a pilgrimage to Rome to benefit from the indulgences on offer to the faithful who pass through the Holy Doors during Jubilee years.
Francis has effectively done away with this custom by ordering cathedrals around the world to open their own Holy Doors. That will happen on Sunday, when Francis himself opens the door at one of Rome's major churches, St John Lateran.
With millions of pilgrims packing the already-crowded streets of Rome, concerns over the consequences of potential attacks are running high with security forces placed on high alert for the duration of the Jubilee.
"There has been no specific threat but the context is troubling," said Rome Prefect Franco Gabrielli, the official in charge of ensuring the Jubilee passes off smoothly and safely.