The cardinals who will choose the next leader of the global Catholic Church were locked in the Sistine Chapel in the Vatican, marking the official start of the conclave to choose Pope Francis' successor.

Shortly after 5:45 p.m. local time (6:45 p.m. Bulgarian time), the master of ceremonies of the conclave announced "Extra omnes" ("All out" in Latin) and then closed the heavy doors of the 15th-century chapel, leaving the 133 "Princes of the Church" to make their choice in secret.

The centuries-old ritual, which is expected to last several days, comes just over two weeks after the death of Pope Francis. The Argentine Jesuit led 1.4 billion Catholics worldwide for 12 years, AFP reported.

Earlier, the cardinal electors gathered in the nearby Pauline Chapel for silent prayer before heading to the 15th-century Sistine Chapel, where tables and chairs are set up beneath Michelangelo's frescoes.

They took a group oath of secrecy before each cardinal approached the altar and took his personal oath before the image of the Italian master's Last Judgment, when, according to Christians, God will return to decide who will go to heaven or hell.

According to video footage broadcast live from the Vatican, they vowed not to reveal what happened during the conclave under threat of excommunication.

This is the largest and most international conclave to date. Cardinals from around 70 countries are participating, many of whom did not know each other before being summoned to Rome after Francis' death.

There is no clear favorite to succeed the charismatic Francis, and the cardinals represent different progressive and conservative traditions within the Church.

However, the challenges facing the 2,000-year-old institution are clear.

The new pope faces diplomatic balancing in a time of geopolitical uncertainty, as well as deep divisions within the Church itself.

In addition, the aftermath of the scandal involving the sexual abuse of children by clergy continues to reverberate, and church pews in the West are increasingly empty. | BGNES