Pope Francis donned the symbols of papal power and vowed to embrace the "poorest" of humanity on Tuesday at a grand inauguration in the Vatican as leader of a troubled Roman Catholic Church.
Some 200,000 pilgrims cheered Latin America's first pontiff in St Peter's Square, waving flags from around the world as Francis promised that his would be a "lowly, concrete and faithful" papacy.
His voice raised for emphasis, 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."
Francis has faced immediate calls to reform the intrigue-filled Roman Curia, the administration of the Church, and take action on the issue of the abuse of children by priests that has been hushed up for decades.
The Argentine pope, who became a voice for the poor during his homeland's devastating economic crisis, has indicated he will be a strong advocate for the dispossessed in an austerity-hit Europe.
Vatican experts have said he will also pursue a more inclusive "collegial" style of leadership together with the cardinals and bishops.
The former archbishop of Buenos Aires, Jorge Mario Bergoglio, has however been a staunch defender of Catholic orthodoxy on issues such as abortion and gay marriage, as well as priestly celibacy.
Francis toured a sun-drenched St Peter's Square in an open-top car to cries of "Long live the pope!"
But Riccardo Monteverde, a 32-year-old working in a homeless shelter in Rome, said he would wait and see whether Francis would live up to expectations.
"We've heard a lot about how Francis wants to help the poor. Only time will tell whether things really will now change for the better," he said.
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.
Some pilgrims wrapped themselves in the Argentine flag and some Brazilian nuns held up a sign reading: "Go Francis! We Will Be With You Wherever You Go!"
The son of an Italian immigrant railway worker, Francis has won hearts in Rome with an informal style which contrasted with Tuesday's pomp.
-- 'Let us care for one another' --
The Vatican said 132 foreign delegations attended.
Bergoglio was the surprise choice at last week's conclave of cardinals to replace 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", warning the world's cardinals against pursuing worldly glories and saying that without renewal the 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.
Zimbabwean President Robert Mugabe flew in, sidestepping an EU travel ban over human rights abuses that does not apply to the Vatican.
Francis later shook hands with Mugabe along with all the other heads of state and government including European and Latin American leaders.
Ecuadorean President Rafael Correa cried upon meeting him, while Chilean President Sebastian Pinera asked him to bless some rosary beads.
Latin America has 40 percent of the world's Catholics in contrast with Europe, where the Catholic population is ageing and declining fast.
Francis is the first non-European pope in nearly 1,300 years and the first Jesuit pontiff, which experts say could give him a more global vision.
Buddhist, Hindu, Jewish, Muslim and Sikh leaders also attended Tuesday's inauguration as did 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 halves of Christendom split.
The Vatican was in security lockdown, with 3,000 officers deployed including sharpshooters on the rooftops and bomb disposal experts.
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.
Francis delivered a message in Spanish, speaking on the phone in comments that were broadcast to tens of thousands on the Plaza de Mayo.
Catholic high school students chanted slogans praising Francis, while seminarians and nuns waved Vatican flags and signs supporting the new pope.
"Please do not forget this bishop, who is far but loves you very much. Pray for me," he said.
Francis also took to Twitter using the @pontifex account set up by his predecessor in an attempt to bring the Catholic message to young people.
"Let us care for one another and let us be loving custodians of creation," he tweeted.