The Argentine native is the first pope from Latin America and the first named for the 13th-century friar St. Francis of Assisi, whose life's work was to care for nature, the poor and most disadvantaged. Echoing the gentleness for which St. Francis is known, the pope said a little bit of tenderness can "open up a horizon of hope."
The Vatican said between 150,000-200,000 people attended the Mass, held under bright blue skies after days of chilly rain and featuring flag-waving fans from around the world. In Buenos Aires, thousands of people packed the central Plaza di Mayo square to watch the celebration on giant TV screens and erupted in joy when Francis called them from Rome, his words broadcast to the crowd over loudspeakers.
"I want to ask a favor," Francis told them. "I want to ask you to walk together, and take care of one another. ... And don't forget that this bishop who is far away loves you very much. Pray for me."
Back in Rome, Francis was interrupted by applause several times during his homily, including when he spoke of the need to protect the environment, serve one another with love and not allow "omens of destruction," hatred, envy and pride to "defile our lives."
Francis said the role of the pope is to open his arms and protect all of humanity, but "especially the poorest, the weakest, the least important, those whom Matthew lists in the final judgment on love: the hungry, the thirsty, the stranger, the naked, the sick and those in prison."
"Today amid so much darkness we need to see the light of hope and to be men and women who bring hope to others," he said. "To protect creation, to protect every man and every woman, to look upon them with tenderness and love, is to open up a horizon of hope, it is to let a shaft of light break through the heavy clouds," he said.
Francis, 76, thrilled the crowd at the start of the Mass by taking a long round-about through the sun-drenched piazza and getting out of his jeep to bless a disabled man. It was a gesture from a man whose short papacy so far is becoming defined by such spontaneous forays into the crowd and concern for the disadvantaged.
The blue and white flags from Argentina fluttered above the crowd, which Italian media initially estimated 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 and allow official delegations through.
Before the Mass began, Francis received the fisherman's ring symbolizing the papacy and a woolen stole symbolizing his role as shepherd of his flock. He also received vows of obedience from a half-dozen cardinals - a potent symbol given his predecessor Benedict XVI is still alive and was reportedly watching the proceedings on TV from the papal retreat in Castel Gandolfo.
A cardinal intoned the rite of inauguration, saying: "The Good Shepherd charged Peter to feed his lambs and his sheep; today you succeed him as the bishop of this church."
Some 132 official delegations attended, including more than a half-dozen heads of state from Latin America, a sign of the significance of the election for the region. Francis 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.
In the VIP section was German Chancellor Angela Merkel, U.S. Vice President Joe Biden, the Argentine President Cristina Fernandez, Taiwanese President Ying-Jeou Ma, Zimbabwe President Robert Mugabe, Prince Albert of Monaco and Bahrain Prince Sheik Abdullah bin Haman bin Isa Alkhalifa, among others. All told, six sovereign rulers, 31 heads of state, three princes and 11 heads of government were attending, the Vatican said.
Francis directed his homily to them, saying: "I would like to ask all those who have positions of responsibility in economic, political and social life, and all men and women of goodwill: let us be protectors of creation, protectors of God's plan inscribed in nature, protectors of one another and of the environment."
Among the religious VIPs attending was the spiritual leader of the world's Orthodox Christians, Bartholomew I, who became 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. Also attending for the first time was the chief rabbi of Rome. Their presence underscores the broad hopes for ecumenical and interfaith dialogue in this new papacy given Francis' own work for improved relations and St. Francis of Assisi.
In a gesture to Christians in the East, the pope prayed with Eastern rite Catholic patriarchs and archbishops before the tomb of St. Peter at the start of the Mass and the Gospel was chanted in Greek rather than the traditional Latin.
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.
"As an Argentine, he was our cardinal. It's a great joy for us," said Edoardo Fernandez Mendia, from the Argentine Pampas who was in the crowd. "I would have never imagined that it was going to be him."
Recalling another great moment in Argentine history, when soccer great Diego Maradona scored an improbable goal in the 1986 World Cup, he said: "And for the second time, the Hand of God came to Argentina."
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.
He has also surprised - and perhaps frustrated - his security detail by his impromptu forays into the crowds.
For nearly a half-hour before the Mass began, Francis toured the square in an open-air jeep, waving, shouting "Ciao!" to well-wishers 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 disabled man held up to the barricade by an aide.
"I like him because he loves the poor," said 7-year-old Pietro Loretti, who attended the Mass from Barletta in southern Italy. Another child in the crowd, 9-year-old Benedetta Vergetti from Cervetri near Rome, also skipped school to attend.
"I like him because he's sweet like my Dad."
In an indication of his devotion to the Virgin Mary, which is common among Latin American Catholics, Francis prayed by a statue of the Madonna at the end of the service.
After the Mass, Francis stood in a receiving line to greet each of the government delegations in St. Peter's Basilica, chatting warmly with each one, kissing the few youngsters who came along with their parents and occasionally blessing a rosary given to him. Unlike his predecessors, he did so in just his white cassock, not the red cape.
On Wednesday, he holds an audience with the visiting Christian delegations. 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 wasn't representing the Anglican Communion, sending instead a lower-level delegation.
Reporter Daniela Petroff contributed.