By Philip Pullella
VATICAN CITY, March 18 (Reuters) - The Vatican was preparing for a huge crowd for Pope Francis's inaugural Mass on Tuesday, an event straddling religion and politics that will bring together European royalty, heads of state and world spiritual leaders.
Hundreds of thousands of people are expected to cram St. Peter's Square and surrounding streets for the Mass to 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.
The Vatican said six sovereigns, including from Belgium and Monaco, will be among the leaders of more than 130 delegations at the Mass.
One significant attendee will be Bartholomew I, the ecumenical patriarch of Orthodox Christianity, the first time a spiritual leader of world Orthodoxy will attend a papal inaugural Mass since the East-West Schism of 1054.
It will be attended by more than 30 delegations representing other Christian Churches as well as representatives of the world's Jewish and Islamic communities.
U.S. Vice President Joe Biden and other Western leaders will be in the same VIP section on the steps of St. Peter's Basilica with Zimbabwe President Robert Mugabe, who is technically infringing a European Union travel ban.
Mugabe has been banned since 2002 because of allegations of vote rigging and human rights abuses but the Vatican is not part of the European Union, allowing him to bend the rules.
The Vatican stressed it does not issue invitations for such events but that they are open to representatives of all nations.
The Vatican meanwhile 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.
The Vatican said some 150 cardinals, including the 114 prelates under 80 years who took part in the conclave that elected him last week, would say the Mass with Francis.
Francis had his first taste on Monday of the diplomatic tightrope he will have to walk, when Argentinian President Cristina Fernandez asked the former Buenos Aires Archbishop Jorge Bergoglio to intervene in a dispute with Britain over the Falkland Islands in the South Atlantic.
"I asked for his intervention on the question of the Malvinas," she told reporters, using the Argentine name for the islands, after having lunch with him in the Vatican shortly after arriving in Rome to attend his Mass.
"I asked for his intervention to avoid problems that could emerge from the militarisation of Great Britain in the south Atlantic," she said.
Fernandez who has led Argentina for six years, has mounted an increasingly vocal campaign to renegotiate the sovereignty of the archipelago, which Britain has resisted, causing a series of diplomatic rows.
British Prime Minister David Cameron said last week that Pope Francis, the first Latin American pontiff, had been wrong to say in 2012 that Britain had "usurped" the disputed islands from Argentina. The year before Bergoglio said that the islands were "ours", a view which most Argentines share. (Editing by Alison Williams)