The crowd who had waited in the rain for hours roared with delight when white smoke billowed out of the Sistine Chapel chimney at 1808 GMT, 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 square until the identity of the new pope was revealed.
The 76-year-old moderate emerged on to the balcony of St Peter's Basilica after a cry of "Habemus Papam!" ("We Have a Pope!").
"I can't believe it! An Argentinian pope!" said Silvia Pastormerlo, a 50-year-old from Argentina.
Fathers propped children up on their shoulders to get a better view of the new pope.
Also in the crowd was an overjoyed Julio Cesar Attaremo, a 42-year-old notary from Santa Fe, in Argentina. "We're very happy and proud, not just for Argentina but for the whole of South America," he said.
"Bergoglio has character. He's very humble and he's someone who really goes out to the people."
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.
"It 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 wait for the smoke ended at 1808 GMT.
People surged into the rain-slicked St Peter's Square and raced towards the Basilica to get a front-row view of the new pope.
Many embraced and kissed each other 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.