VATICAN CITY - Pope Francis opened the great bronze doors of St. Peter's Basilica on Tuesday to launch his Holy Year of Mercy, declaring that mercy trumps moralizing in his Catholic Church.

Francis stood in prayer on the threshold of the basilica's Holy Door then walked through it, the first of an estimated 10 million faithful who will pass through over the course of the next year in a rite of pilgrimage dating back centuries.

A thin and frail Emeritus Pope Benedict XVI walked through the doorway right after Francis, gingerly negotiating the two steps with the help of a cane and his longtime assistant. It was a rare outing for the 88-year-old Benedict, whose historic resignation led to Francis' election.

Some 5,000 extra police, carabinieri, and soldiers have been deployed around Rome, and a no-fly zone imposed on its skies, to protect the pilgrims who are flocking to Rome for the yearlong celebration.

Security was heightened after the Vatican was listed as a possible target following the Paris attacks.

Francis launched the 12-month jubilee to emphasize what has become the leitmotif of his papacy: showing the more merciful and welcoming side of the church.

"We have to put mercy before judgment, and in any event God's judgment will always be in the light of his mercy," he told an estimated 50,000 under rainy skies for his Mass opening the jubilee.

The Vatican's Holy Door, to the right of the basilica's main entrance, is decorated with 16 bronze panels depicting the redemption of man's sin through mercy. Passing through it is meant to symbolize the pilgrimage of life's journey and the sacrifices endured.

After Francis and Benedict walked through the doorway, the first throngs of pilgrims followed.

In a sign that Francis himself was taken aback by Benedict's frailty when he saw him at the Holy Door, he asked the throngs of pilgrims in the piazza to send their prayers for his "good health." The crowd responded with cheers and applause.

Holy Years are generally celebrated every 25 to 50 years, and over the centuries they have been used to encourage the faithful to make pilgrimages to Rome to obtain an "indulgence" - the ancient church tradition related to the forgiveness of sins.

The last jubilee was in 2000, when St. John Paul II ushered in the church's third millennium and some 25 million pilgrims flocked to Rome.

In typical Francis fashion, the pope made clear from the start that he wanted this Holy Year to be a more sober occasion. For the first time, the pope instructed all cathedrals around the world to open their Holy Doors to pilgrims to encourage the faithful to mark the jubilee at home.