Cardinals choosing the next leader of the global Catholic Church were locked into the Vatican's Sistine Chapel on Wednesday, marking the formal start of the conclave to elect a successor to Pope Francis.

Shortly after 17:45 (15:45 GMT), the conclave's master of ceremonies declared "Extra omnes" – "Everybody out" in Latin – and then shut the heavy doors of the 15th-century chapel, leaving the 133 "Princes of the Church" to make their choice in secrecy.

The centuries-old ritual, which is expected to last several days, comes just over a fortnight after the death of Pope Francis, the Argentine Jesuit who led the world's 1.4 billion Catholics for 12 years.

The cardinal electors had earlier gathered in the nearby Pauline Chapel in silent prayer before proceeding to the 15th-century Sistine Chapel, where tables and chairs have been laid out beneath Michelangelo's frescoes.

They took a group oath of secrecy before each cardinal approached the altar to utter his personal vow in front of the Italian master's depiction of the Last Judgement, when Christians believe God will return to decide who goes to heaven or hell.

According to a live video feed produced by the Vatican, they took a vow not to reveal what happened in the conclave, on pain of excommunication.

The conclave is the largest and the most international ever, with cardinals from around 70 countries – many of whom did not know each other before being summoned to Rome following Francis's death.

There is no clear frontrunner to succeed the charismatic Francis, with the cardinals representing a range of progressive and conservative traditions within the Church. (AFP)