Argentina's Jorge Mario Bergoglio was elected Pope Francis I today, becoming the church's first Latin American pontiff after a conclave to elect a leader of the world's 1.2 billion Catholics.

The 76-year-old conservative emerged from the balcony of St Peter's Basilica to the cry of "Habemus Papam!" ("We Have a Pope!"), as tens of thousands of pilgrims clambered over barriers and broke down in tears, overcome with emotion after suspenseful prayer vigils worldwide.

White smoke earlier billowed from the Sistine Chapel and the bells of St Peter's Basilica rang out, signalling the election had taken place after five rounds of voting in the Vatican -- one more than when Benedict XVI was elected in 2005.

Bergoglio is the first Jesuit to become pope and is believed to have been the runner-up in 2005.

The first wisps of smoke in the evening sky prompted cries of "Long live the pope!" from pilgrims clutching rosaries and waving flags in the square, where the image of the tiny copper chimney was projected onto four giant screens.

Bergoglio, who is the 266th pope in the Catholic Church's 2,000-year history, retired to a chamber known as the "Room of Tears" immediately after the nomination to don his papal vestments and then prayed in the Pauline Chapel.

Bells pealed in churches across Italy to celebrate the announcement and residents of Rome could be seen racing to the floodlit 17th-century Vatican plaza, running out of their homes and cafes to reach the square in time.

Cardinals have been locked up behind the Vatican walls and cut off from the outside world since yesterday, meeting in a sublime Renaissance chapel swept for recording devices and installed with scramblers to prevent any communication.

The historic election after Benedict's abrupt resignation last month was being followed around the world on live television as as well as through social media and smartphone apps -- this is the first ever tweeted conclave.

"I didn't think I would cry but I guess the adrenalin's taking over!" said Rebecca Hine, a student from Canada who had waited two days in the rain.

"I'm overjoyed!" said a tearful Veronica, a nun from Botswana. "I'm so emotional I can hardly speak!"

A breathless Ruud, a 31-year-old Dutch tourist, said: "We were having dinner nearby and heard a roar, and raced here to see for ourselves."

Benedict's eight-year papacy was riven by scandals and the new pope will face immediate challenges -- stamp his authority on the Vatican machinery and try to bring back a Catholic flock that is deserting churches across the West.

Benedict's style was often seen as too academic and he was never as popular as his predecessor. Many of the cardinals have called for the new pope to be a better communicator, able to reach out particularly to young people.

Conclaves are usually only held after a pope dies and are sometimes decades apart -- the last one was in 2005, the one before that 1978. A popular Italian expression for things that happen very rarely is "at every death of a pope".

The 85-year-old Benedict broke with tradition, becoming the first pontiff to resign since the Middle Ages. He has said he will retire to a former nunnery inside the Vatican -- an unprecedented and delicate situation for the Church.

In one of his last acts as pope, he issued a decree allowing cardinals to bring forward the date of a conclave in cases of papal resignation -- a move seen by many as potentially setting a precedent for future ageing pontiffs.

The scandal of hushed-up sexual abuses of children by paedophile priests going back decades has also cast its shadow over the conclave.

The US group SNAP (Survivors Network of those Abused by Priests) called for over a dozen cardinals to be excluded from the conclave either for covering up abuses or for making tactless remarks about the scandals.

Vatican spokesman Federico Lombardi today defended the cardinals and accused SNAP and other activists of showing "negative prejudices".