The Pro Eligendo Romano Pontefice ("For the Election of the Roman Pontiff") mass is presided by Angelo Sodano, the elderly dean of the College of Cardinals, and is also open to non-voting cardinals -those aged more than 80.
After the mass, expected to last about two hours, the so-called Princes of the Church are due to head for the Domus Sanctae Marthae (Santa Marta), a modern guesthouse inside the Vatican's grounds, before taking part in a procession into the Sistine Chapel, where they will be locked in from the outside world until a new pontiff is elected.
The first round of voting is due to take place in the late afternoon, with Vatican spokesman Federico Lombardi anticipating black smoke, signalling no pope has yet been elected.//DPA