VATICAN CITY: 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.

Inspired by St Francis of Assisi, whose name he chose, Francis said Popes must "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 intrigue-filled 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.

Vatican experts have said he will also pursue a more inclusive "collegial" style of leadership together with the cardinals and bishops.

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 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.

The son of an Italian immigrant railway worker, Francis has won hearts in Rome with an informal style which contrasted with Tuesday's pomp.

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.

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.

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 temporarily before he plans to move into a secluded former convent within the Vatican.

Francis is due to meet his predecessor at Castel Gandolfo on Saturday.

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 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.

Zimbabwean President Robert Mugabe also 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.

German Chancellor Angela Merkel, the daughter of a Protestant pastor, found the mass "emotional", according to a German government spokesman.

She was said to be "impressed by the simple and direct way of speaking" of the new Pope.

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.

Mexican President Enrique Pena Nieto told broadcaster Televisa before Tuesday's mass: "The Pope's world leadership can have great importance in what happens in Latin America."

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 and there was an unprecedented attendance by 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 the 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."