Elector cardinals from across the world celebrate the Mass "Pro Eligendo Romano Pontifice", led by Cardinal Giovanni Battista Re in St Peter's Basilica a few hours before the start of the conclave. Some 133 cardinal electors gathered at the Vatican from five continents will be shut into the Sistine Chapel to elect the next head of the world's 1.4 billion Catholics, successor to Pope Francis. The Mass Pro Eligendo Romano Pontifice is the last rite to be celebrated publicly before the Church's 267th pope is presented to the world. IMAGES