Tens of thousands of people in St Peter's Square on Wednesday celebrated with Argentina Latin America's first pope -- Jorge Mario Bergoglio -- with many hoping he will revolutionise the Church.

Newly elected Pope Francis was met with a roar from an ecstatic crowd as he appeared in his papal whites for the first time on a balcony high up on the basilica's facade and stood as if humbled before the wild applause and tears in the crowd.

After waiting in the rain for hours, people roared with delight when white smoke billowed out of the Sistine Chapel chimney at a few minutes after 7:00 pm local time, signalling a new pope had been chosen after five rounds of voting.

There was then an agonising hour-long wait for the faithful from all over the world, who stood shoulder to shoulder in the darkness until the identity of the new pope was revealed.

The 76-year-old emerged on to the balcony of St Peter's Basilica to a cry of "Habemus Papam!" ("We Have a Pope!") and "long live the pope!"

The new pontiff looked overwhelmed, and took a moment to compose himself before greeting the crowd, which fell silent and held a collective breath as they waited for his first words.

"Dear brothers and sisters, good evening!" he said with a smile, sparking rapturous applause from nuns, priests, school children and pilgrims who gazed up at the papal balcony.

"I can't believe it! An Argentinian pope!" said Silvia Pastormerlo, a 50-year-old from Argentina, while Jose Garcia, 24, from Mendoza, said "I'm really pleased he's an Argentinian! My mum burst into tears when it was announced."

Fathers propped children up on their shoulders to get a better view of the new pope.

"He has such a kind face! He spoke to us as if he was one of us, I think it is an excellent sign!" said Sarah Hopper, a Catholic student from America.

There was joy too from Europeans in the crowd.

"I'm overwhelmed, I never would have though it would be an Argentinian. I thought the Church was too traditional and Eurocentric for that," said Greta Hinder, 24, from the Netherlands.

-- Revolutionary moment --

"I think it's a massive step forward, this is a revolutionary moment," she said, as her friends hugged each other around her.

One Italian couple cracked open a bottle of champagne.

Benedetta Vitellano, 32, said: "We were at home nearby and we heard a commotion and rushed to see.

"It's amazing, we're toasting to a new beginning."

The long wait for the white smoke had been punctuated by a seagull landing on top of the modest copper chimney on the chapel roof, which many people said was a good omen.

Then there was a sudden outpouring of joy when the smoke appeared in the night air just as the rain stopped.

People surged into St Peter's Square and raced towards the Basilica to get a front-row view of the new pope.

Many embraced and kissed and phoned their loved ones, saying "they've done it!" and "we have a pope!"

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

Catholicism in Latin America is growing rapidly and there had been calls for the new pope to come from the southern hemisphere for the first time.

There had also been calls for the new pontiff to be the first to adopt the name Francis, to draw upon the legacy of St Francis of Assisi, the 13th-century founder of the Franciscan Order and a symbol of asceticism.

"I prayed for a pope Francis, and I got one!" said Saverio, an Italian architect who had spent the conclave period holding up a sign in St. Peter's Square calling for a pope Francis.

"I think the Church has realised we need to go back to our roots. This is a fantastic sign!" he said.