VATICAN CITY (AP) - In a historic gathering steeped in intrigue, cardinals from six continents assembled Monday for their first conclave of the new millennium to elect a pope who will inherit John Paul II's mantle and guide the world's 1.1 billion Roman Catholics into a new era.

Representing 52 countries, the 115 crimson-robed ''princes'' of a church stung by priest sex-abuse scandals and an exodus of the faithful celebrated a midmorning Mass at St. Peter's Basilica before sequestering themselves in the Sistine Chapel late Monday afternoon.

There, seated atop a false floor hiding electronic jamming devices designed to thwart eavesdroppers, they were to take an oath of secrecy, hear a meditation from a senior cardinal and decide whether to take a first vote or wait until Tuesday.

''The new pope has already been chosen by the Lord. We just have to pray to understand who he is,'' Florence Cardinal Ennio Antonelli, considered by some to be a dark-horse candidate, told believers who gathered for Sunday Mass at his titular church in Rome.

In his homily Monday morning, Cardinal Joseph Ratzinger - a powerful Vatican official from Germany often mentioned as a leading candidate to become the next pope - spoke in unusually blunt terms against ''a dictatorship of relativism'' - the ideology that there are no absolute truths.

''Having a clear faith, based on the creed of the church, is often labeled today as a fundamentalism,'' Ratzinger said. ''Whereas relativism, which is letting oneself be tossed and 'swept along by every wind of teaching,' looks like the only attitude acceptable to today's standards.

''We are moving toward a dictatorship of relativism which does not recognize anything as for certain and which has as its highest goal one's own ego and one's own desires.''

Thousands of pilgrims and tourists were expected to converge on St. Peter's Square to watch the chapel chimney for the white smoke that ultimately will tell the world that the church's 265th pontiff has been elected. The famous stove in the chapel also will bellow black smoke to signal any inconclusive round of voting.

''I feel really cool being here,'' said Kathy Mullen, 49, a writer from Beverly, Mass., among the hundreds of pilgrims who lined up early on a sunny morning to pass through metal detectors on their way into the basilica.

Although the conclave could last for days, a pope could be chosen as early as Monday afternoon if the red-capped prelates opt to begin casting ballots after their solemn procession from the Vatican's Apostolic Palace to the chapel.

If they decide to hold off a day, they will hold four rounds of voting - two in the morning, two in the afternoon - on Tuesday and every day until a candidate gets two-thirds support: 77 votes. If they remain deadlocked late in the second week of voting, they can vote to change the rules so a winner can be elected with a simple majority: 58 votes.

Vatican spokesman Joaquin Navarro-Valls said smoke from burned ballot papers enhanced by special chemicals likely could be seen at around noon and around 7 p.m. on each day of voting by the cardinal electors, all of whom are under age 80. At some point soon after the new pope is chosen, the Vatican also will ring bells.

On Sunday, the cardinals moved into the super-secure Domus Sanctae Marthae, the $20 million hotel that John Paul had constructed inside Vatican City so the cardinals could rest in comfort in private rooms between voting sessions. Swiss Guards, their brightly colored uniforms covered by dark rain gear, saluted the prelates as they were whisked to the residence in limousines.

The daily La Stampa said cardinals gearing up for a stressful stretch of days had packed CD players and headphones in their bags along with prayer books and snacks to nibble on in their rooms.

Conspicuously missing from their quarters were cell phones, newspapers, radios, TVs and Internet connections - all banned in new rules laid down by John Paul II to minimize the chances of news influencing their secret deliberations and to prevent leaks to the outside world. The Vatican's security squad swept the chapel for listening devices, and cooks, maids, elevator operators and drivers were sworn to secrecy. Excommunication is a possible punishment for any indiscretions.

No conclave in the past century has lasted more than five days, and the election that made John Paul II pope in October 1978 took eight ballots over three days. He died April 2 at 84 after a pontificate that lasted more than 26 years, history's third-longest papacy.

Cardinals faced a choice that boiled down to two options: an older, skilled administrator who could serve as a ''transitional'' pope while the church absorbs John Paul's legacy, or a younger dynamic pastor and communicator - perhaps from Latin America or elsewhere in the developing world where the church is growing - who could build on the late pontiff's popularity over a quarter-century of globe-trotting.

The prelates agreed after John Paul's funeral not to talk publicly about the process, but the world's news media have been rife with speculation centering on about two dozen candidates considered ''papabile,'' Italian for ''pope material.''

The name with the biggest buzz was Ratzinger's, a powerful Vatican official from Germany who was to recite an opening prayer in Latin that the voters be guided ''in our hearts, in love and in patience.''