Vatican City - Pope Francis inaugurated on Tuesday the Jubilee of Mercy, an 11-month festival that offers the faithful a general pardon for their sins, with the 500-year-old ritual of the opening of the holy door at St Peter's Basilica.
Francis led a 09:30 (08:30 GMT) mass with top cardinals in a heavily policed St Peter's Square. A relatively low turnout of about 50 000 people followed the ceremony, braving light rain and stringent security checks, which caused queues.
At the end of the service, Francis kickstarted the Jubilee by opening the holy door - the first of five from the right in the basilica's facade - in a gesture symbolically offering a path of redemption to pilgrims.
"Is this the Lord's door?" asked the leader of the world's 1.2 billion Catholics, as he walked up to the heavy bronze gate, which took several pushes to prize open. "Open unto me the gates of justice," he said.
Delivering a homily in the open-air mass, the pope hailed the Jubilee, also known as a holy year, as a "gift of grace". He said it was wrong to think of God as a punisher, rather than a forgiver, of sin.
"We have to put mercy before judgment," Francis said. "The history of sin can only be understood in the light of God's love and forgiveness. Were sin the only thing that mattered, we would be the most desperate of creatures."
In a rare outing, Pope Emeritus Benedict XVI hugged his successor Francis before the holy door ritual, and was the first to follow him into the basilica. Looking frail, the 88-year-old walked with a stick, aided by his secretary, Archbishop Georg Gaenswein.
The ceremony, which ended with Francis and a procession of cardinals praying by the altar where St Peter's remains are buried, was broadcast worldwide, with German arthouse cinema director Wim Wenders contributing to filming.
At noon, Francis delivered an Angelus message in which he urged the faithful to not be afraid of seeking God's pardon. He was later due to pay respect to a Virgin Mary shrine near the Spanish Steps in central Rome, and visit the Basilica of St Mary the Major.
After sunset, a light show of nature photography was to project the works of world renowned photographers such as Sebastiao Salgado and Steve McCurry on the facade of St Peter's Basilica, in a nod to climate change talks under way in Paris.
The Jubilee is to run until November 20 2016.
In a first, it is to be celebrated not just in Rome. All seeking redemption are invited to make a pilgrimage to any of the holy doors that will be opened in cathedrals and other places of worship around the world, and walk through them.
The December 8 start date falls on the Feast of the Immaculate Conception, an important Catholic festival, on the 50th anniversary of the Second Vatican Council, which ushered in major reforms in the 1960s, and on the 1 000th day of Francis' pontificate.
Fears of terrorist attacks cast a shadow on preparations for the Jubilee, especially after US intelligence publicly warned in the wake of the November 13 Islamist strikes in Paris that St Peter's was another possible target.
Cardinal Secretary of State Pietro Parolin, the highest Vatican authority after the pope, told Italian daily Corriere della Sera: "We cannot let ourselves be paralysed by fear, which after all is what the authors of terrorist attacks want."
The first Catholic Jubilee was first celebrated in 1300, but its origins go back to Jewish tradition. The word jubilee comes from the Hebrew jobel, meaning ram's horn, which was used as a trumpet to announce the beginning of festivities.
The Jubilee of Mercy is the 30th holy year in Catholicism.
Catholic jubilees were initially meant to take place every 100 years, but the time interval was gradually shortened, until it was set at 25 years in the 15th century. In addition, popes can declare "extraordinary" jubilees, like the one starting on Tuesday.