Thick black smoke emerged on Wednesday from the chimney of the Sistine Chapel in a sign that cardinals had failed to elect a new head of the Catholic Church in their first conclave vote.

Tens of thousands of people gathered in St Peter's Square to await the smoke, which came around three hours and 15 minutes after the 133 cardinals were closed in.

The prelates will now withdraw to the Santa Marta guesthouse, where they are staying for the election, before starting to vote again on Thursday.

Cardinals were called back to Rome following the death of Pope Francis on April 21 after 12 years as head of the world's 1.4 billion Catholics.

Under a centuries-old ritual, those aged under 80 vote in secrecy in the Sistine Chapel until one of them secures a two-thirds majority – 89 votes – to be elected pope.

Locked away to avoid distraction, their only means of communicating the outcome is by burning their ballots with chemicals to produce smoke.

It is black if there is no decision, white if they have a new pope. (AFP)