A huge crowd converged on St. Peter's Square on Tuesday to see Pope Francis celebrate his inaugural Mass amid a wave of hope for a renewal of the scandal-plagued Roman Catholic Church.

The Mass, which was due to start at 9:30 a.m. (0830 GMT) will formally install Francis as the new leader of the world's 1.2 billion Roman Catholics.

The crowd may be the biggest in Rome since more than 1.5 million people came to the city for the beatification of the late Pope John Paul II on May 1, 2011.

"We are originally from Argentina and we wanted to be here today because Pope Francis is from our home town. We were so proud when he was elected. We travelled overnight so we could be here today," said Cirigliano Valetin, 51, an electrician who works in Salerno in southern Italy.

"He is a simple, humble person, he is not like the untouchable popes, he seems like someone normal people can reach out to," said Valetin, who is originally from Buenos Aires

Six sovereigns, U.S. Vice President Joe Biden and other leaders as well as heads of many other faiths will be among the 130 delegations on the steps of the famous basilica.

Among them will be Ecumenical Patriarch Bartholomew from Istanbul, the first time the spiritual head of Orthodox Christians attends a Roman pope's inaugural Mass since the Great Schism between western and eastern Christianity in 1054.

The former Buenos Aires Cardinal Jorge Bergoglio has aroused enormous enthusiasm and interest in the Catholic world by the modest way he has assumed a post that was modelled after a Renaissance monarchy and carries titles such as "Vicar of Jesus Christ" and "Supreme Pontiff of the Universal Church".

In the six days since his election, he has referred to himself only as Bishop of Rome, the position from which his authority flows, and hinted he plans to reduce Vatican centralism and govern in consultation with other bishops.

Signals of change

Vatican watchers expect him to send further signals of change by using a simpler liturgy than retired Pope Benedict, who preferred a more baroque style.

The ceremony has also been shortened to two hours after a three-hour service in 2005 when Benedict began his papacy.

Francis started the day with a ride around St. Peter's Square to greet the crowds. He will receive the visiting political leaders in the basilica after the Mass.

On Wednesday, Francis will receive more than 30 delegations representing other Christian churches, as well as the Jewish, Muslim, Buddhist, Sikh and Jain religions, a Vatican spokesman said.

He will address foreign ambassadors to the Vatican on Friday and have lunch with Benedict -- their first meeting since the conclave -- on Saturday before leading celebrations the next day for Palm Sunday, the first day of Holy Week leading to Easter.

Francis had his first taste of the diplomatic challenges of the papacy when on Monday, Argentinian President Cristina Fernandez asked him to support Buenos Aires in a dispute with Britain over the Falkland Islands in the South Atlantic.

A Vatican spokesman had no comment on the request.

He will also find himself greeting an international pariah, Zimbabwe President Robert Mugabe, who has been under a European Union travel ban since 2002 because of allegations of vote rigging and human rights abuses.

The Vatican is not part of the European Union, allowing him to travel.

The Vatican on Monday revealed the new pope's coat of arms - similar to the one he used as Archbishop of Buenos Aires - with symbols representing Jesus, Mary and Joseph.

Symbols of the papacy have been added behind it, including two keys which signify the Biblical passage in which Jesus told St. Peter he would give him "the keys of the kingdom" of heaven.

The motto on his coat of arms is "Miserando atque eligendo" (Having had mercy, he called him), which comes from a meditation by the Venerable Bede, an English monk in the 8th century, on a passage of the Gospel in which Jesus calls St. Matthew to be an apostle.

In various sermons and comments since his surprise election last Wednesday, the pope has urged people to be more merciful and not to be so quick to condemn the failings of others.

The papal ring chosen by Francis is made of gold-plated silver and depicts St. Peter holding the keys, the Vatican said.

The ring and a pallium - a liturgical vestment worn around the neck and made of lamb's wool to symbolise what the Church teaches is Jesus' role as shepherd of souls - will be placed on the spot where St. Peter is believed to be buried.

Both will be kept on the tomb during the night and the pope will wear them before the Mass.