Pope Francis has warned in his first Mass that the troubled Catholic Church risked becoming little more than a charity with no spiritual foundations if it failed to undergo renewal.
Addressing the cardinals who elected him as Latin America's first pope, the 76-year-old Argentinian said on Thursday the Church could "end up a compassionate NGO", using an Italian word that can also mean "pitiful".
"I would like all of us after these days of grace to have the courage to walk in the presence of the Lord," Francis said, amid the splendour of the Sistine Chapel.
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He warned the cardinals against "the worldliness of the Devil".
"Walking, building and confessing are not so easy. Sometimes there are tremors," the pope said, in a homily that will be scrutinised for clues to the style of his leadership.
The new head of the world's 1.2 billion Catholics, who was formerly known as Cardinal Jorge Mario Bergoglio, had begun his reign by meeting people in Rome and laying a bouquet of flowers in homage to the Virgin Mary in a basilica.
The pope also prayed at the altar of St Ignatius of Loyola, the founder of the Jesuit order to which he belongs.
He returned to the priests' quarters where he stayed before the conclave and settled his own bill.
The election of the son of a railway worker, who was considered an outsider, was met with widespread surprise and expressions of hope for change in a Church riven by scandal and internal conflict.
It was also seen as recognition of the Church's power in Latin America, which now accounts for 40 percent of the world's Catholics, in contrast to its decline in Europe.
"The choice of Bergoglio shows that the Church is determined not to remain in mourning for the crisis in Europe but has opened its doors to the revitalising energy of Catholicism's biggest continent," Vatican expert Luigi Accatoli told the AFP news agency.
Blase Bonpane discusses the Argentine church's past
Al Jazeera's Lucia Newman, reporting from the Argentine capital of Buenos Aires, said that Francis revealed what he would have done if elected as pope, in an interview with an Argentinian journalist, shortly before he left to the Vatican.
"One of the things he said he would have done is to eradicate the corruption from the 'guilded palaces' of the Vatican. He also said that the Vatican Bank needs to be cleaned up and made transparent immediately, and that 'everyone knew' who the corrupt cardinals were in the Vatican," Newman said.
"So he clearly plans to do some serious housecleaning, now that he is in charge," she added.
Projecting an image as a simple man of the people, the pope chose to name himself after St Francis of Assisi, the 13th-century saint who shunned the riches of his family to devote himself to God and the poor.
The Vatican revealed that, for the ride back to the conclave lodgings after Wednesday's election, Francis shunned the papal limousine with the "Vatican City State One" number plates, opting instead to board a minibus with the cardinals.
It was in keeping with his image as a man who as archbishop of Buenos Aires chose to live in a modest apartment rather than the official residence and took buses to work.