With the ritual cry of Extra omnes" - "Everybody out" - the bronze doors of the Sistine Chapel swung shut yesterday and the Roman Catholic cardinals inside began deliberating on their choice of a 283rd pope.

The world will not again see the III prelates until they elect a successor to Paul VI. While the process could go on for days, the conventional wisdom here holds that the choice of a new pope could come Sunday afternoon or Monday.

At least a dozen cardinals are "papabili," men regarded as possible pontiffs. But the cardinal who ascends the papal throne must gain the ballots of 75 of his colleagues, two-thirds plus one. Reducing so large a group to pass so high a barrier could be a long process.

Yesterday afternoon's ceremony in a Sistine Chapel overheated by television lights - several cardinals were seen mopping their brows - marked the opening of the secret conclave. The first vote, however, will not be taken until Saturday morning.

The cardinals plan to hold four votes daily - two in the morning and two in the afternoon. All the outside world will know of their deliberations will be contained in the puff of smoke emerging from the Sistine Chapel stove and visible in St. Peter's Square below.

Black smoke will mean no choice has yet been made. The waiting crowd will learn a pontiff has been elected when the chimney sends up a white signal.

Yesterday morning, the world at large, or at least the few thousand people who packed St. Peter's could see for themselves an extraordinary sight - the 111 cardinals in scarlet robes white mitres or red skull caps, all celebrating a mass.

Two by two, the cardinals passed in splendid procession, past eager nuns, tourists and journalists. The prelates arrayed themselves in front of Bernini's Throne of St. Peter in the apse of the Basilica, a marvel of four Baroque statues of outsized saints who lead the eye to a dazzling bronze sunburst that all but explodes through the cupola.

Four at a time, the cardinals came to the altar table to eat the wafer and sip the wine that symbolizes the body and blood of Christ in Catholic doctrine. The nuns below flocked to the railings to receive the same communion from priests in white surplices.

After the mass, the cardinals walked back through St. Peter's to the Vatican Palace, and several exchanged greetings with friends behind the railings. Cardinal Reginald Delargey of New Zealand was heard telling one man, "I'll see you Monday."

Italian television interrupted it coverage of an amateur baseball game yesterday to record the opening of the conclave. The cardinals, again in scarlet robes, wearing pectoral crosses and scarlet birettas over scarlet skull caps, were seen sitting in the overly hot Pauline Chapel. They were reading what appeared to be hymn or prayer books. Behind them was Michelangelo's "Conversion of St. Paul," a masterpiece the public does not get to see.

Then Cardinal Jean Villot, administrator of the church until a new pope is elected, signaled to the Sistine Chapel's choir, men and boys in a white surplices. The choir sung in Latin, "Come Holy Spirit," a phrase with particular significance for the election.

Catholic belief holds that the Holy Spirit infuses itself in each elector, guiding the deliberations. But as theologians here wryly acknowledge, man is an imperfect vessel and more concrete considerations - nationality, view towards ecumenism, notions of the church's role in the Third World, administrative ability - will also influence votes.

Led by the choir and a priest bearing a cross, the cardinals filed into the Sistine Chapel, two by two, and took the seats at which they will mark their ballots. With Violet, they prayed for spiritual guidance, and the doors were closed.

Before they retired to their chambers in the Vatican Palace, each cardinal swore an oath of secrecy about the deliberations. Villot then preached a homily about the importance of their task. He also called in the Swiss Guards, doctors and others of the 75 non-electors who will serve the prelates in their sealed-off area. Each of these persons, too, was sworn to secrecy.

Last night, the cardinals, it was assumed, were praying. Some no doubt were also discussing how to reduce the large field of candidates when the first ballots are cast starting at9:30 Saturday morning.