Pope Francis knelt at the tomb of St Peter and donned the symbols of papal power at a sumptuous inauguration on Tuesday, vowing to embrace the "poorest, the weakest" of humanity.
Nearly 200,000 pilgrims cheered Latin America's first pontiff in St Peter's Square, waving flags from around the world as the newly elected popes promised that his would be a "lowly, concrete and faithful" papacy.
In an address strongly influenced by the teachings of St Francis of Assisi, the saint he has chosen as his inspiration, he urged world economic and political leaders not to "allow omens of destruction and death to accompany the advance of this world!"
His voice raised in emotion, the 76-year-old Francis said a pope must "embrace with tender affection the whole of humanity, especially the poorest, the weakest, the least important."
"Amid so much darkness, we need to see the light of hope," said the Argentinian, after touring a sun-drenched St Peter's Square in an open-top car to cries of "Long live the pope!"
The former Buenos Aires Cardinal Jorge Bergoglio was a fervent critic of the International Monetary Fund and unregulated market capitalism -- a stance that could make him an important voice in an austerity-hit Europe.
At the ceremony, the 265th successor to St Peter received from his cardinals the papal pallium -- a lambswool strip of cloth that symbolises the pope's role as a shepherd and has red crosses to represent the wounds of Jesus Christ.
The "Fisherman's Ring" bestowed on him by Angelo Sodano, dean of the college of cardinals, is a personalised signet ring traditionally worn by popes in honour of St Peter -- a fisherman.
"With Pope Francis, the Church will be closer to the people and to the modern world," said Rodrigo Grajales, a 31-year-old Colombian priest.
Francis gave the thumbs-up as he toured the square, stopping to kiss babies and getting out of the car at one point to bless a disabled man.
"Go Francis! We Will Be With You Wherever You Go!" read a sign held up by a group of Brazilian nuns in St Peter's Square.
Sister Rosa, an elderly Italian nun, said she expected the pope would be "another St Francis on Earth for love, goodness, poverty and humility".
The Vatican said there were between 150,000 and 200,000 people present at the ceremony.
The son of an Italian immigrant railway worker, Francis has already won hearts in Rome with a disarmingly informal style which contrasted with Tuesday's pomp and ceremony.
The Vatican said 132 foreign delegations attended.
Bergoglio was the surprise choice at last week's conclave of cardinals to find a successor to 85-year-old Benedict XVI, who last month brought a sudden end to a papacy that had often been overshadowed by scandal, saying he was too old to carry on.
He was the first pope to resign since the Middle Ages.
Francis has called for a "poor Church for the poor", warning the world's cardinals against pursuing worldly glories and saying that without deep spiritual renewal the Roman Catholic Church would crumble "like a sand castle".
The arrival of world leaders has presented him with a first diplomatic headache in the form of a request from compatriot President Cristina Kirchner of Argentina to mediate in a row with Britain over sovereignty of the Falkland Islands.
Francis is still haunted by criticism from left-wingers at home for failing to speak out against the excesses of Argentina's military rule during the dictatorship of the 1970s and 1980s.
The Chinese government also said it would not be sending any representatives after Taiwanese President Ma Ying-jeou said he was attending.
Zimbabwean President Robert Mugabe flew in, sidestepping an EU travel ban over human rights abuses that does not apply to the Vatican.
Latin America was heavily represented at the inauguration of the first non-European pope in nearly 1,300 years, with the presidents of Argentina, Brazil, Chile, Costa Rica, Mexico and Paraguay all in attendance.
German Chancellor Angela Merkel, Spanish Prime Minister Mariano Rajoy, French Prime Minister Jean-Marc Ayrault and European Union leaders were also present.
Leaders of the Eastern Catholic Rite were also there, including Bartholomew I, the Ecumenical Patriarch of Constantinople.
Vatican radio said it was the first time a patriarch of Constantinople had attended an inauguration since 1054 when the eastern and western halfs of Christendom split.
The Vatican was in security lockdown for the event, with 3,000 officers deployed including sharpshooters on the rooftops and bomb disposal experts.
Church leaders have urged Francis to move quickly to reform the intrigue-filled Roman Curia, the central administration of the Roman Catholic Church, and his appointments in the coming weeks will be closely watched.
Francis has indicated he will press for a friendlier faith that is closer to ordinary people and for social justice, although the moderate conservative is unlikely to change major tenets of Catholic doctrine.
Vatican experts say he has also signalled he will pursue a more inclusive "collegial" style of leadership together with the cardinals and bishops.
Vast crowds also gathered on the other side of the Atlantic outside the Buenos Aires cathedral to dance and sing as they watched the inauguration.
Catholic high school students chanted slogans praising Francis, while seminarians and nuns waved Vatican flags and signs supporting the new pope.
"This pope has awakened deep emotions within me, not only because he's from Argentina, but because of his warmth as a person," Celia Farias, 33, told AFP.
"As a Catholic, it has renewed my faith."