VATICAN CITY (AFP) - Pope Francis has warned that the troubled Catholic Church risks becoming little more than a charity with no spiritual foundations if it fails to undergo renewal.

The 76-year-old Argentinian told the cardinals who elected him as Latin America's first pope that the Church could "end up a compassionate NGO".

"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 in his first mass on Thursday, amid the splendours of the Sistine Chapel.

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.

On Friday he is scheduled to meet the cardinals at a formal audience in the Vatican.

The election of the son of an Italian emigrant railway worker, who was considered a rank outsider, was met with widespread surprise and expressions of hope for change in a Church riven by scandal and internal conflict.

His elevation was also seen as recognition of the Church's power in Latin America, which now accounts for 40 per cent of the world's Catholics, while it is in decline in Europe.

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.