"Pray for me," the new pontiff, dressed in the white robes of a pope for the first time, urged the crowd, smiling warmly.
NEWSMAKER - Argentina's Pope Bergoglio http://in.reuters.com/article/2013/03/13/pope-francis-bergoglio-idINDEE92C0FM20130313
Pictures - Meet the new pope http://in.reuters.com/news/pictures/slideshow?articleId=INRTR3EXVC
The choice of Bergoglio, who is the first Latin American and first Jesuit pope, was announced by French cardinal Jean-Louis Tauran with the Latin words "Annuntio vobis gaudium magnum. Habemus Papam" ("I announce to you a great joy. We have a pope.")
Francis has became the 266th pontiff in the Church's 2,000-year history at a time of great crisis, with the church under fire over a child sex abuse scandal and torn by infighting in the Vatican bureaucracy.
"I hope he changes all the luxury that exists in the Vatican, that he steers the church in a more humble direction, something closer to the gospel," said Jorge Andres Lobato, a 73-year-old retired state prosecutor.
Although a conservative, Francis is seen as a reformer and was not among the small group of frontrunners identified before the election. The Jesuit order to which he belongs was founded in the 16th century to serve the pope. It is best known for its work in education and the intellectual prowess of its members.
Bergoglio is known as a humble man who leads an austere and sober life without ostentation, travelling by public transport and living in a small apartment outside Buenos Aires.
He is a moderate who is willing to challenge powerful interests and is deeply concerned about the social inequalities in Argentina and elsewhere in Latin America. He has had a sometimes difficult relationship with President Cristina Fernandez and her late husband and predecessor Nestor Kirchner.
Francis has spoken out strongly against gay marriage, denouncing it in 2010 as "an attempt to destroy God's plan".
He was born into a middle-class family of seven, his father an Italian immigrant railway worker and his mother a housewife.
Replacing Pope Benedict, who resigned last month, he overturned one of the main assumptions before the election, that the new pope would be relatively young.
Bergoglio is the oldest of most of the possible candidates and was barely mentioned in feverish speculation about the top contenders before the conclave.
He is the first non-European pope since Syrian-born Gregory III in the eighth century, and the third successive non-Italian pontiff.
The Vatican said his inaugural mass would be held on March 19.
U.S. President Barack Obama said the election of Francis "speaks to the strength and vitality of a region that is increasingly shaping our world."
"PRAY FOR BENEDICT"
In brief remarks from the balcony of St. Peter's, Francis called on the faithful to pray for Benedict and said the Church was setting off on a "journey of fraternity, of love, of trust."
It seemed the cardinal electors "went to the end of the world" to find him, he said.
The Vatican said Francis would visit Benedict soon at his temporary home in the summer papal residence outside Rome.
Thousands of people sheltering from heavy rain under a sea of umbrellas had occupied the square all day to await the decision and the crowd swelled as soon as the white smoke emerged.
They cheered wildly and raced towards the basilica as the smoke billowed from a narrow makeshift chimney and St Peter's bells rang.
The tens of thousands in the square cheered even more loudly when Francis appeared, the first pontiff to take that name. "Viva il Papa (pope)," they chanted.
The election was enthusiastically welcomed in Latin America. "I am happy because another European pope would be like eating the same bread every day," said Mexico City cab driver Martin Rodriguez.
"We're happy because we have a new pope and because the choice of a Latin American shows that the Church is opening, is now focused on the entire Church. It's not just a church only focused on Europe," said Leonardo Steiner, general secretary of the national conference of Brazilian bishops.
Frontrunners at the conclave had included Brazilian Odilo Scherer, and Italy's Angelo Scola, who would have returned the papacy to traditional Italian hands after 35 years of the German Benedict XVI and Polish John Paul II.
The decision by cardinal electors sequestered in a secret conclave in the Sistine Chapel came sooner than many experts expected because there were several frontrunners before the vote to replace Pope Benedict.
The cardinals faced a thorny task in finding a leader capable of overcoming crises caused by priestly child abuse and a leak of secret papal documents that uncovered corruption and rivalry inside the Church government or Curia.
Francis will head a Church also shaken by rivalry from other churches, the advance of secularism, especially in its European heartland, and allegations of scandal at the Vatican bank.
The series of crises is thought to have contributed to Benedict's decision to become the first pontiff in 600 years to abdicate.
Graphics on the conclave http://link.reuters.com/ryw56t
RIVAL TO RATZINGER IN 2005
Bergoglio was a moderate rival candidate at the 2005 conclave to the conservative Cardinal Joseph Ratzinger, who went on to become Benedict.
Italian media say he impressed cardinals in pre-conclave meetings where they discussed the Church's problems.
Reserved and humble, Francis does not fit the profile of an active preacher that many cardinals had previously said they were seeking. He studied chemistry before joining the priesthood nearly a decade after losing a lung to respiratory illness.
"He's very humble, I heard that in Buenos Aires he used to take public transport, have an apartment and cook for himself. The fact that he chose the name Francis means a lot. It means we will have a humble, simple pope close to the poor people. But it was a big surprise," said Jules Charette, 54, a Canadian lawyer in St. Peter's Square.
Bands from the Italian armed forces and the Vatican's own Swiss guard army paraded in front of the basilica before the new pope appeared.
The secret conclave began on Tuesday night with a first ballot and four ballots were held on Wednesday. Francis obtained the required two thirds majority in the fifth ballot.
Following a split ballot when they were first shut away amid the chapel's Renaissance splendour on Tuesday evening, the cardinal electors held a first full day of deliberations on Wednesday. Black smoke rose after the morning session to signal no decision.
The previous four popes were all elected within two or three days.
Seven ballots have been required on average over the last nine conclaves. Benedict was clear frontrunner in 2005 and elected after only four ballots.
In preparatory meetings before the conclave, the cardinals seemed divided between those who believe the new pontiff must be a strong manager to get the dysfunctional bureaucracy under control and others who are looking more for a proven pastoral figure to revitalise their faith across the globe.
Apart from Brazil's Scherer and Italy's Scola, a host of other candidates from numerous nations had also been mentioned as potential popes - including U.S. cardinals Timothy Dolan and Sean O'Malley, Canada's Marc Ouellet and Argentina's Leonardo Sandri.
But the frontrunners list never mentioned Bergoglio.
(Additional reporting by Naomi O'Leary, Catherine Hornby, Crispian Balmer and Tom Heneghan in Rome, Dave Graham in Mexico City, Ana Flor in Brasilia, Nicolas Misculin and Juliana Castilla in Buenos Aires; Writing by Barry Moody; Editing by Keith Weir, Alastair Macdonald and Giles Elgood)