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, AFP reports.

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.