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 pope in St Peter's Square, as Francis promised a "lowly, concrete and faithful" papacy.
The pope said he would aim to "embrace with tender affection the whole of humanity, especially the poorest, the weakest, the least important".
The 76-year-old head of the world's 1.2 billion Catholics has faced immediate calls to reform the Vatican bureaucracy and take action against the ongoing scourge of child sex abuse by priests.
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 suffering under deep austerity cuts in Europe.
His exhortation to world leaders to protect "God's creation" was seen by Vatican watchers as a subtle reference to the Church's anti-abortion stance.
As archbishop of Buenos Aires, Jorge Mario Bergoglio was a staunch defender of Catholic doctrine on this and other "life" issues such as contraception and euthanasia.
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 to the Catholic flock.
The "Fisherman's Ring" bestowed on him is a personalised signet ring traditionally worn by popes in honour of St Peter -- a fisherman by trade.
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," he tweeted. The account's followers in Spanish have surged to more than one million, bringing the total for all languages to nearly four million.
-- 'Pray for me' --
The son of an Italian immigrant railway worker, Bergoglio was the surprise choice at last week's conclave to replace 85-year-old Benedict XVI, who last month ended a papacy often overshadowed by scandal, saying he was too old to carry on.
The first pope to resign since the Middle Ages watched the mass on television from the papal summer residence of Castel Gandolfo near Rome where he is living.
Francis is due to meet his predecessor at Castel Gandolfo on Saturday. He called Benedict after the inauguration.
Francis has said he wants 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 132 foreign delegations presented Francis 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 the Falkland Islands.
Francis is also 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.
But Fray Horacio, a Franciscan vicar in Argentina, told AFP that two Catholic priests and a layman assassinated during the "Dirty War" are being considered for sainthood under a process launched in 2011 by Bergoglio.
Among the delegations on Tuesday were several European and Latin American leaders.
German Chancellor Angela Merkel, the daughter of a Protestant pastor, said she was impressed by Francis's "simple and direct way of speaking", according to a German government spokesman.
Mexican President Enrique Pena Nieto told broadcaster Televisa: "The pope's world leadership can have great importance in what happens in Latin America".
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.
Among the delegations from world religions there was also Bartholomew I, the ecumenical patriarch of Constantinople.
It was the first time a patriarch of Constantinople, spiritual leader of the world's 250 million Orthodox Christians, had attended an inauguration since 1054 when the eastern and western halves of Christendom split.
Vast crowds also gathered on the other side of the Atlantic outside Buenos Aires cathedral to dance and sing as they watched the inauguration.
In a phone call broadcast on the square, Francis told them: "Please do not forget this bishop, who is far but loves you very much. Pray for me."