Updated 7.36pm

Robert Francis Prevost became the first pope from the United States on Thursday, picking the papal name Leo XIV after cardinals from around the world elected him leader of the world's 1.4 billion Catholics.

Tens of thousands of people packed in St Peter's Square cheered as Prevost appeared on the balcony of St Peter's Basilica, waving with both hands, smiling and bowing.

"Peace by with you," he told the crowds, before beginning his first address, where he called for "building bridges".

The 69-year-old native of Chicago, who spent years as a missionary in Peru, was made a cardinal by Pope Francis in 2023.

White smoke from the Sistine Chapel had billowed into the sky on the cardinals' second day of voting to announce his election in a secret conclave, while the bells of St Peter's Basilica and churches across Rome rang out.

Crowds of people rushed towards the square to watch the balcony of the basilica, which has been fitted out with red curtains for the first address to the world by the 267th pope.

Cardinal Mario Grech with fellow cardinals as Pope Leo XIV addressed the public from the balcony.

The new pontiff, who succeeds Argentine reformer Pope Francis, was introduced in Latin with his chosen papal name.

"It's an amazing feeling," said an elated Joseph Brian, a 39-year-old chef from Belfast in Northern Ireland, who came with his mother to Rome for the spectacle. 

"I'm not an overly religious person but, being here with all these people just blew me away," he told AFP as people around him jumped up and down in excitement.

There were euphoric scenes as one priest sat on someone's shoulders waving a Brazilian flag and another lifted a heavy crucifix into the air in jubilation.

'Habemus Papam'

"Habemus papam, woooo!" howled Bruna Hodara, 41, from Brazil, echoing the words to be spoken on the balcony as the new pope is introduced.

She, like others, recorded the historic moment on her phone, as others waved flags and cried out "Viva Il Papa!" - "Long live the pope!" in Italian.

"It's a once in a lifetime opportunity to be here to see the pope. It's really special... I'm excited!" said Florian Fried, a 15-year-old from Munich, in Germany.

Pope Francis died last month aged 88 after a 12-year papacy during which he sought to forge a more compassionate Church -- but drew anger from many conservatives with his progressive approach.

An onlooker flying the Maltese flag celebrates the election of a new pope. Photo: Reuters

The new pope now faces a momentous task: as well as asserting his moral voice on a conflict-torn global stage, he must try to unite a divided Church and tackle burning issues such as the the continued fall-out from the sexual abuse scandal.

Some 133 "Princes of the Church" from five continents -- the largest and most international conclave ever -- began voting on Wednesday afternoon.

Sworn to secrecy, on pain of excommunication, their only means of communicating their progress to the outside world was by sending up smoke through the chimney of the Sistine Chapel.

On Wednesday evening and then again on Thursday lunchtime, the smoke was black, eliciting disappointed sighs from the tens of thousands watching.

Carabinieri take position on the parvis of St Peter's basilica before the first appearance of the new Pope. Photo: AFP

But on Thursday afternoon just after 6:00 pm (1600 GMT) the smoke emitted was white, confirming that the Catholic Church has a new spiritual leader.

It was unknown how many ballots it took to elect the new pope, but it followed recent history in wrapping up in less than two days.

In 2005, Benedict XVI, a German theologian, was elected in four ballots and Francis, in 2013, was elected in five ballots.

While the details of the election will forever remain secret, the new pope had to secure at least two-thirds of votes to be elected.

A nun prays as she waits for the appearance of the newly elected Pope after white smoke rose from the chimney of the Sistine Chapel. Photo: AFP

The question was whether to choose a pastor or diplomat, a liberal or conservative, someone versed in the Curia - the Church's governing body - or a relative outsider from areas of the world where Catholic faith is thriving.

Before the cardinals were locked into the Sistine Chapel Wednesday, their dean Giovanni Battista Re urged them to choose someone able to protect the Church's unity.