The blue and white flags from Francis' native Argentina fluttered above the crowd that Italian media estimate could reach 1 million. Civil protection crews closed the main streets leading to the square to traffic and set up barricades for nearly a mile (two kilometers) along the route to try to control the masses.
For nearly a half-hour, Francis toured the square in an open-air jeep, waving and occasionally kissing babies handed up to him as if he had been doing this for years. At one point, as he neared a group of people in wheelchairs, he signaled for the jeep to stop, hopped off, and went to bless a man held up to the barricade by an aide.
The installation Mass is a simpler affair than the 2005 ceremony that launched Pope Benedict XVI's papacy, in keeping with Francis' sober style, but it is still grand enough to draw 132 official delegations and religious leaders from around the world.
Among the VIPs is the spiritual leader of the world's Orthodox Christians, Bartholomew I, who will become the first patriarch from the Istanbul-based church to attend a papal investiture since the two branches of Christianity split nearly 1,000 years ago. His presence underscores the broad hopes for ecumenical and interfaith dialogue in this new papacy.
But it is Francis' history of living with the poor and working for them while archbishop of Buenos Aires that seems to have resonated with ordinary Catholics who say they are hopeful that Francis can inspire a new generation of faithful who have fallen away from the church.
"I think he'll revive the sentiments of Catholics who received the sacraments but don't go to Mass anymore, and awaken the sentiments of people who don't believe anymore in the church, for good reason," said Judith Teloni, an Argentine tourist guide who lives in Rome and attended the Mass with a friend.
Francis has made headlines with his simple style since the moment he appeared to the world on the balcony of St. Peter's Basilica, eschewing the ermine-lined red velvet cape his predecessor wore in favor of the simple papal white cassock, then paying his own bill at the hotel where he stayed prior to the conclave that elected him pope.
During Tuesday's Mass, Francis will receive the woolen pallium, or stole symbolizing his role as shepherd of his flock, and also the simple gold-plated silver fisherman's ring that is a symbol of the papacy.
A wax cast of the ring was first presented to Pope Paul VI, who presided over the second half of the Second Vatican Council, the 1962-65 meetings that revolutionized the church. Paul never wore it but the cast was subsequently made into the ring that Francis chose among several other more ornate ones.
Francis will receive each of the government delegations in St. Peter's Basilica after the Mass, and then hold an audience with the visiting Christian delegations on Wednesday. He has a break from activity on Thursday; a gracious nod perhaps to the fact that the Archbishop of Canterbury, Justin Welby, is being installed that day in London.
As a result, Welby won't be representing the Anglican Communion at Tuesday's installation Mass for Francis, sending instead a lower-level delegation. All told, six sovereign rulers, 31 heads of state, three princes and 11 heads of government will be attending, the Vatican said.
More than a half-dozen Latin American presidents are attending, a sign of the significance of the election for the region. Francis, named after the 13th century friar known for his care of the most disadvantaged, has made clear he wants his pontificate to be focused on the poor, a message that has resonance in a poverty-stricken region that counts 40 percent of the world's Catholics.
For Jews, Orthodox and other religious leaders, the new pope's choice of Francis as his name is also important for its reference to the Italian town of Assisi, where Pope John Paul II began conferences encouraging interfaith dialogue and closer bonds among Christians.
Follow Nicole Winfield at www.twitter.com/nwinfield