The world's oldest body of electors, the College of Cardinals, enters the walled-off precincts of the Vatican Palace today to choose a new spiritual leader for 600 milliion Roman Catholics.

Until they choose a new pope, successor to Paul VI, the 111 cardinals will pray, confer, eat and sleep all but cut off from the temporal world outside.

Their only signal to it will be a puff of smoke sent up twice daily from their voting chamber, the magnificent Sistine Chapel. If the smoke is black, the world will know that the cardinals have not yet agreed. White smoke will announce that one cardinal has received the necessary 75 votes, two-thirds plus one.

The cardinals begin voting Saturday, with two ballots scheduled in the morning and another two in the afternoon. If no pope is chosen, the process will be repeated on Sunday and Monday. Tuesday is set aside for prayer and meetings to break any deadlock. If one persists, the cardinals will vote in seven consecutive sessions, break for a day of prayer, and - if necessary - hold another seven sessions.

The Italian cardinals considered the strongest candidates are Sebastiano Baggio, 65; Giovanni Benelli, 57; Paulo Bertoli, 70; Sergio Pignedoli, 67; Paulo Bertoli, 70; Sergio Pignedoli, 67, and Ugo Poletti, 64. They are usually described as Paulinists, meaning that they are likely to carry on Paul's work of a church more open to the larger world.

The major non-Italian candidates - who are considered less likely to be elected - are Eduardo Pironio, 57, of Argentina, a prelated identified with the concern of the Latin American church with oppresive military regimes; Franz Koenig, 72, of Austria, a strong supporter of the church's opening to other faiths; Jan Willebrands. 85, of the Netherlands, a noted philosopher and ecumenist; and Basil Hume, 56, of Britain, a priest whose spiritual quality has forcibly impressed the most skeptical.

Today's opening of the conclave is largely given over to housekeeping and the first round of informal soundings behind the plywood walls erected to close the collegge from the world.

The cardinals will assemble in the Sistine Chapel, taking their close-packed seats at 12 simple tables each about 25 feet long. Unlike the pomp of the past when each elector sat on canopied throne, the cardinals will sit on simple wooden chairs whose austerity is relieved only by a red velvet seat.

In scarlet robes and red birettas, the cardinal will face each other arrayed in two rows on each side of the chapel's long axis. They will sit under superb frescos of scenes from the life of Moses on one side and the life of Jesus on the other.

Jean Cardinal Villot, the Vatican cemerlengo or administrator of the church between popes, will then read the election rules laid down by Paul. They reflect his preoccupation secrecy, even providing for technicians to sweep the sealed-off quarters for electronic listening devices.

Each of the 111 cardinals will then "promise and swear to observe with the greatest fidelity . . . the secret concerning everything everything that in any way relates to the election of the Roman pontif and concerning what takes place in the conclave."

After taking the oath, the cardinals will retire to their rooms, somewhat fancifully described as cells.

They are, in fact, Vatican offices in the palace loggias and most are large, high-ceilinged chambers.

Their furnishings are spartan, however. A typical room boast an iron cot, no rugs or pictures on the walls and a simple night table. On this are a roll of toilet paper, a box of tissues and a bar of soap.

On another table, a folder of paper of pens permits the taking of notes. A prie-dieu stands under a small image of Christ on the cross. Perhaps the most pratical touch is four or five armchairs to allow groups of electors to caucus.

The cardinals will also see each other outside the voting chamber at meal time. They will dine in a room once inhabited by a Borgia pope, now hung with contemporary religious paintings. Intimate conversation, however, will be hard. The cardinals will sit on inexpensive, plastic chairs at three 60-foot-long tables.

Their food will be prepared and served by nuns from the Order of St. Martha. They are among the 75 non-electors in the sealed-off quarters. There are also doctors, firemen, druggists and others necessary to serve the elderly cardinals.

Groceries will be delivered to the conclave through a large, cylindrical drum inserted in one of the walls. A cutaway section will swing through the wall to pick up deliveries, swing back to deposit the load. Another smaller cylinger will open out to pick up papers. The two hollow drums and a small grill for speaking are the conclave's only links to the Vatican's unsealed area.

To underline the secretary, all windows have been chained down, the bottom panes have been painted white and blinds have been drawn. The conclave will deliberate in airlines chambers under weak, electric light.

Even so, theologians here are predicting that an early conclusions is unlikely. The cardinal are described as realistic men, skilled at compromise. But there are simply too many plausible possibilities, and several ballots will be needed, it is said, to shake down the list.

Given the solemn voting process, it is not even sure that the 111 cardinals, with an average age of 68, will be able to hold the scheduled four daily votes.

Each will receive a card saying in Latin, "I elect as supreme pontiff." A blank space is left for a name. One by one, the cardinals will carry their ballots to the marble altar beneath Michaelangelo's overwhelming "Last Judgment." The cardinal kneels, swears that "my vote is given to the one who before God I consider should be elected," and deposits his ballot in a silver chalice.

After each round, the ballots are counted and then burned in a gray, cast iron stove that sits in a corner of the entrance to the Sistine Chapel. A tall gray stacks runs from the stove to a window just below the high ceiling and sticks out above the roof. A chemical stick, either one producing black and or one producing white smoke, is added to the pyre. It is this signal that crowds gathering in St. Peter's Square will watch, starting Saturday.

They may have a long wait for white smoke because there is no obvious choice and a dozen names are floating around Vatican City. In part, this reflect the varying technological and political strains among the cardinals. It is also a measure of the geographical diversity that Paul gave the college.

For the first time in anyone's memory here there is a genuine chance of a non-Italian pope. The last was Adrian VI, from Holland, who was unlucky enough to reign at the height of Martin Luther's rebellion in 1522 and 1523.

Only 27 of the 111 cardinals are Italian. But there are powerful reasons for continuing the tradition and choosing one as pope. One scholar points out that the Italian church needs all the support it can muster at a time when Communists are making their weight felt in the nation's political life. A non-Italian pope, he argues, would weaking the hold of the church here.

In addition, he observe, the pope must administer a vital bureaucracy, the 2,000 functionaries in the Curia. Again, it is said, an Italian pope is more likely to operate this machine smoothly.

Those who have talked privately with cardinals say there is a wide area of agreement on the characteristics desired in the new pope. He must, they say, make no retreat from the ecumenical moves toward unity with other religions. He should continue and expand Paul's effort to give bishops a greater voice in church affairs. He should continue the church's growing concern for and involvement with the poor, especially in the Third World.

Finally, he must be a pastoral pope, although liberals and conservatives assign different meanings to this word. For liberals, it means close identification with parishioners. For conservatives, it means strengthening Catholic identity, renewing faith and tradition.