Pope Francis baptized 32 babies in the Sistine Chapel on Sunday and told their mothers, including one who was married in a civil service rather than in church, to have no qualms about breast-feeding them there.

Unlike his predecessors, who usually delivered long and theology-laden homilies at the yearly baptism event, the pope offered a brief, improvised homily of some 300 words centred on the children.

"Today the choir will sing but the most beautiful choir of all is the choir of the infants who will make a noise. Some will cry because they are not comfortable or because they are hungry," he said in a familiar, relaxed tone to the parents.

Michelangelo's frescoes in the Sistine Chapel are some of the world's most celebrated works of art. The ceiling depicts the creation of man and the altar wall shows a severe God at the Last Judgment. But the pope told the mothers not to feel intimidated by the surroundings.

"If they are hungry, mothers, feed them, without thinking twice. Because they are the most important people here," he said, speaking in the same room where he was elected on March 13 as the first non-European pope in 1,300 years.

Francis said in an interview last month that mothers should not feel uncomfortable breastfeeding during his ceremonies.

In another apparent first in the Vatican, the parents of one of the babies, 7-month-old Giulia Scardia, at the ceremony were not married in church but only at a civil service in a town hall — meaning their marriage is technically not recognized by the Catholic Church.

But the pope has said several times since his election that the Church must not make children of couples in irregular situations feel like second-class faithful, and he agreed to baptize Giulia Scardia into the faith.

"We decided to get married very quickly," Giulia's mother Nicoletta told the Corriere della Sera newspaper. "We were in a hurry and there was no time to organize a church ceremony. Maybe we will do it sometime."

Sunday's service was the latest example of the more down-to-earth style Francis has introduced in the Vatican.

He has renounced the spacious papal apartments in the Apostolic Palace used by his predecessors and lives is a small apartment in a Vatican guest house. Francis uses the palace only to receive heads of state and to address crowds from one of its windows overlooking St. Peter's Square.

He has also given up the papal limousine and is driven around Rome in a Ford Focus, sometimes sitting in the front seat next to the driver.

Baptism is the sacrament at which infants or converts are initiated into the Christian faith. Francis poured water on the foreheads of the infants as part of the ritual.