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 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."
President Barack Obama offered "warm wishes" to Pope Francis and said the selection speaks to the strength and vitality of the Americas.
"On behalf of the American people, Michelle and I offer our warm wishes to His Holiness Pope Francis," Mr Obama said.
"As a champion of the poor and the most vulnerable among us, he carries forth the message of love and compassion that has inspired the world for more than two thousand years that in each other we see the face of God.
"As the first pope from the Americas, his selection also speaks to the strength and vitality of a region that is increasingly shaping our world, and alongside millions of Hispanic Americans, those of us in the United States share the joy of this historic day"
The Archbishop of Canterbury, Archbishop Justin Welby, offered his welcome, adding: "We wish Pope Francis every blessing in the enormous responsibilities that he has assumed on behalf of Roman Catholics around the world.
"I look forward to meeting Pope Francis, and to walking and working together to build on the consistent legacy of our predecessors. May the love of Christ unite us, and intensify our service in a genuine and fruitful ecumenism that can be a blessing for the Body of Christ throughout the world.
"Pope Francis is well known as a compassionate pastor of real stature who has served the poor in Latin America, and whose simplicity and holiness of life is remarkable."
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.
"I look forward to continuing co-operation between the United Nations and the Holy See, under the wise leadership of His Holiness Pope Francis.
"We share many common goals - from the promotion of peace, social justice and human rights, to the eradication of poverty and hunger - all core elements of sustainable development."
He added: "I am certain that His Holiness will continue to build on the legacy of his predecessor, Pope Benedict XVI, in the promotion of inter-faith dialogue which is at the heart of the Alliance of Civilisations initiative."
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."
He told the BBC: "I think he will bring a new kind of style to the Catholic Church. Here is a man who has experienced over many years, especially in his own country but also elsewhere, something of the love for poor people.
"His own simplicity of life, I think will be a great example to people.
"For many people this may be a surprise election but for me it is inspired and I am very very happy, not only for the Catholic Church, but for the world."
The cardinal said it was wrong to ponder over why, if Pope Francis was the wrong choice in 2005, when he reportedly came second, he was the right man now.
"This time the cardinals have elected Pope Francis and they were very happy to do so.
"The question doesn't arise - one conclave is different from another."
The Bishop of Wakefield, the Rt Rev Stephen Platten, who is chairman of the Governors of the Anglican Centre, in Rome, and former co-secretary of the Anglican Roman Catholic International Commission (ARCIC), welcomed the election of Pope Francis I.
He said: "It is exciting for all of us that the new Holy Father is from the New World.
"It is exciting too that he brings with him a profound theological background formed within the Jesuit family.
"It was encouraging, too, to hear of his commitment to simplicity in his own lifestyle. As a Franciscan myself I was delighted to hear of the new Pontiff's name.
"The governors of the Anglican Centre look forward to continuing to work with the Holy See in support of the Archbishop of Canterbury's relations with the Roman Catholic Church and indeed, the relations of the Anglican Communion worldwide."