Crowds of people including Political and religious leaders from around the world have gathered at St Peter's Square, in the Vatican to celebrate the inaugural Mass of Pope Francis’ papacy.The first pontiff from the Americas, Francis has suggested he will take a more modest approach than predecessors. He was elected by a conclave of cardinals last week to succeed Benedict XVI, who resigned on health grounds.The BBC's Katya Adler, reports from the Vatican, that nuns, priests and papal tourists have been queuing for hours early on Tuesday, with some pushing and squabbling as they waited for police to open the barricades to St Peter's Square. Papal ring The Pope is scheduled to leave his temporary residence at Casa Santa Marta and tour St Peter's Square on a papal vehicle for about 15 minutes before going to St Peter's Basilica to don his vestments and stop at the tomb of St Peter.He will then go into The Mass scheduled for 09:30 am (GMT) in the main square, co-celebrated by around 180 clergymen, including Adolfo Nicolas, the superior general of Pope Francis' Jesuit order.After delivering a homily, Francis will be presented with his papal pallium made of lambs' wool and the "fisherman's ring" bearing the image of St Paul holding two keys. The Pope has asked for some of the pomp to be stripped away. The ring is second-hand and is made of silver plated in gold, not the solid gold worn by his predecessor.Pope Francis has also chosen to keep the same coat of arms he had as archbishop of Buenos Aires, with the addition of the papal symbols - a gilded mitre and crossed gold and silver keys.The list of attendees for the Mass includes US Vice-President Joe Biden and the spiritual head of the Orthodox Church, Patriarch Bartholomew. He will be the first Orthodox patriarch to attend a papal inauguration Mass since the two branches of Christianity split more than 1,000 years ago.Thirty-three groups of guests from various Christian Churches are expected in total, according to the Vatican, along with 16 delegations from Jewish communities, as well as representatives of other faiths. Argentine vigils Before his election last week, Pope Francis was Cardinal Jorge Mario Bergoglio of Buenos Aires, and Argentine President Cristina Fernandez de Kirchner will attend the Mass.Thousands of people held overnight vigils across Argentina in preparation for Tuesday's Mass. The event will be broadcast on giant screens set up outside the cathedral on Plaza de Mayor, the main square in Buenos Aires.Correspondents say that in his first few days in office, Pope Francis has been striking an informal and spontaneous tone, preferring anecdotes and off-the-cuff remarks to scripted speeches. Speaking on Saturday, he emphasised that he wanted "a poor Church, for the poor".He is the first pontiff to take the name of Francis, choosing it to honour St Francis of Assisi, the 13th-Century son of an aristocrat who spurned a life of luxury to live with and for the poor.At the end of a Mass he celebrated on Sunday, he waited outside the church and greeted people as they left, like a parish priest, asking many of them to "pray for me". Later, just a few minutes after delivering the Angelus prayer, Pope Francis sent his first Tweet as pontiff, writing: "Dear friends, I thank you from my heart and I ask you to continue to pray for me. Pope Francis."