Hundreds of thousands of people are expected to attend the mass laden with rituals and spectacular imagery, which will begin with a tour of the famous Vatican plaza by the Argentine pope after his election last week.
Still haunted by criticism at home for failing to speak out against the excesses of Argentina’s military rule, the former archbishop of Buenos Aires, Jorge Mario Bergoglio, has already won hearts in Rome with a disarmingly informal style.
Bergoglio was a surprise choice at a 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 jovial Francis has said he chose his papal name in honour of the mediaeval Italian saint St Francis of Assisi and has called for a “poor Church for the poor”, warning the world’s cardinals against pursuing worldly glories.
The son of an Italian emigrant railway worker from a working-class quarter of Buenos Aires has been effusive in a way that is unusual in the Vatican, kissing pilgrims and doing impromptu walkabouts.
The Vatican has said security guards will have to adapt to the new style.
The arrivals have already presented Francis, 76, 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.
The Chinese government has also said it will not be sending any representatives after Taiwanese President Ma Ying-jeou said he was attending.
Zimbabwean President Robert Mugabe also flew in, sidestepping an EU travel ban over human rights abuses in his country that does not apply to the Vatican.
Latin America will be heavily represented at the mass by the first non-European pope in nearly 1,300 years, with the presidents of Argentina, Brazil, Chile, Costa Rica, Mexico and Paraguay all expected to attend.
The Vatican will be in security lockdown for the event, with 3,000 officers deployed including sharpshooters on the rooftops and bomb disposal experts.
The day’s events will begin with a tour of St Peter’s Square by the new pope starting a few minutes before 9:00 am (0800 GMT), after which Francis will pray at the tomb of St Peter, who is considered the first pontiff in Catholic tradition.
Francis will then receive from his cardinals the pallium – a lambswool strip of cloth that symbolises the pope’s role as a shepherd – and the “Fisherman’s Ring”, a personalised signet ring representing his authority.
The ring is named in honour of St Peter, a fisherman by trade.
The mass proper is expected to begin at around 9:30 am and will include a homily by Francis, who has often strayed from prepared texts with off-the-cuff jokes, anecdotes and passionate exhortations for spiritual renewal.
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.
The Church is being challenged on many levels around the world – including growing secularism in the West, the rise of radical Islam and the ongoing scandal of abuses of children by priests that were hushed up for decades. ref: news/wire services/toi