Pope Francis pushed opened the huge bronze Holy Door of St Peter's Basilica in Rome to launch the Catholic Church's "Year of Mercy".
Tens of thousands attended a Mass in St Peter's Square for the start of the Pope's "revolution of tenderness".
It took place place amid tight security with extra police and soldiers deployed, and a no-fly zone imposed.
Under the year's theme of mercy, the Pope has said priests can absolve women who have had abortions.
During the jubilee celebrations, one of the most important events in the Roman Catholic Church, pilgrims travel to Rome and religious sites around the world.
At the end of the Mass, Francis opened the basilica's Holy Door. He said that by passing through it, Catholics should take on the role of the Good Samaritan.
It is the first time the Holy Door has been opened since the Great Jubilee in the 2000 called for by St John Paul II. It has been bricked up since then.
Emeritus Pope Benedict XVI, 88, attended Tuesday's event.
Jubilee years are rooted in the Old Testament tradition of freeing slaves and prisoners once every 50 years, a concept that died out within Judaism but was taken up by Pope Boniface VIII for the Catholic Church in 1300.
Pilgrimages to Rome were at the heart of the original jubilee years, and attracted hundreds of thousands of pilgrims to the city, many willing to pay for "indulgences" - the eradication by the Church of the spiritual debt arising from sin.
It was a tradition that not only contributed copious cash to the Vatican's coffers, but also contributed to the theological turmoil that led to the establishment of rival Protestant churches across much of northern Europe.
The last Jubilee was called by St John Paul II to mark the millennium, and this Holy Year of Mercy starts on the Feast of the Immaculate Conception on 8 December 2015 and will end on the Feast of Christ the King on 20 November 2016.
What is the Catholic Year of Mercy? - by Caroline Wyatt, BBC Religious affairs correspondent
Italian security forces are on high alert following recent attacks in Paris and San Bernardino, California.
Visitors to St Peter's Square had to pass through metal detectors and under go bag and body checks.
Announcing the extraordinary jubilee in March, the Pope said the Holy Door was a "Door of Mercy, through which anyone who enters will experience the love of God who consoles, pardons and instils hope".
For the first time, he has instructed churches and cathedrals to take part in the tradition of the Holy Door, to help Catholics mark the jubilee at home rather than coming to Rome.
Pope Francis has long signalled his wish to change the Church's approach from condemnation of wrongdoing to a Church that is more forgiving and understanding of its flock, our correspondent says.
This extraordinary jubilee year is seen as a practical way of giving expression to that wish.
Pope Francis took many by surprise when he announced in September that, as part of the jubilee, parish priests across the world would be allowed to absolve repentant women who asked for forgiveness for having an abortion, even though Church teaching still terms abortion a grave sin.