The election of Pope Francis I has been welcomed as "a momentous day".
Minutes after the new pope addressed the crowd gathered in St Peter's Square, Rome, Prime Minister David Cameron gave his reaction.
He tweeted: "A momentous day for the 1.2bn Catholics around the world as His Holiness Pope Francis I is appointed the 266th Bishop of Rome."
Archbishop Vincent Nichols, President of the Catholic Bishops' Conference of England and Wales, expressed his joy at the news: "On behalf of the Catholic Bishops' Conference of England and Wales, I wish to express my great joy at the news of the election of Cardinal Jorge Mario Bergoglio as the 265th Successor of St Peter and Supreme Pontiff of the Catholic Church.
"As he begins his new Apostolic ministry, as the Bishop of Rome and Shepherd of the universal Church, Pope Francis I may be assured of the prayers and loving support of the Catholic community throughout England and Wales."
UN secretary-general Ban Ki-moon said: "I offer my heartfelt congratulations to Cardinal Jorge Mario Bergoglio of Buenos Aires, Argentina, who has taken the name Pope Francis on his assumption of the papacy, and to all Catholics across the world on this momentous occasion."
Scotland's First Minister Alex Salmond said: "My warmest congratulations go to Pope Francis on his election, which will be welcomed by Scotland's Catholic community and by others around the world.
"I wish him well in his time in the Papacy, and his first message as Pope - urging greater bonds of understanding and co-operation among peoples and nations - is one that should resonate around the globe."
Cardinal Cormac Murphy-O'Connor, the former head of the Roman Catholic Church in England and Wales, said the new Pope was an "inspired" choice.
"He is a humble, gentle and very intelligent and spiritual man," he told Channel 4 News. "I think that Pope Francis is going to be a blessing for the Catholic Church and for the world. Many will think it is a surprise choice, for me it is an inspired choice."