Sequestered behind Vatican walls until they reach agreement on the next spiritual leader of the world's 1.1 billion Catholics, their only communication with the outside world until then will be black smoke signals emitted from a narrow chimney atop the Sistine Chapel.

The first black smoke signal emerged shortly after 8pm at the end of a tradition-laden day of ceremony, prayer and suspense.

Initially, it appeared to the 20 000 people massed on Saint Peter's Square that the first wisps were white, indicating a successful vote, but the signal soon turned emphatically black, indicating the first of what could be many inconclusive rounds of balloting.

The 115 cardinal electors will henceforth vote four times a day until they reach the required two-thirds majority.

Special mass to pray for God's guidance

The prelates embarked on their momentous task on Monday with a special mass in St Peter's Basilica led by Cardinal Joseph Ratzinger to pray for God's guidance.

Ratzinger, a close confidant of John Paul II who is among leading candidates to succeed him, used the platform of his homily to defend Church doctrine and condemn the rise of transient ideologies.

Later on Monday, the cardinals processed solemnly into the Sistine Chapel for the formal start of the conclave, taking an oath of secrecy and foreswearing politicking.

The master of liturgical ceremonies, Archbishop Piero Marini, then intoned the time-honoured words "Extra Omnes", the order for all those not taking part to leave the chapel.

Secret part of the conclave

The secret part of the conclave then began with a 20-minute meditation delivered by a Czech cardinal, Tomas Spidlik, the second of two that John Paul II decreed in 1996 should be given to help cardinals make up their minds.

Last week, the prelates were warned not to be "ambitious".

The Sistine Chapel, normally packed with tourists admiring the magnificent frescoes including Michelangelo's Last Judgment, has been swept for bugs and a special system will scramble any mobile telephone communications.

With the cardinals now cloistered in their conclave, Vatican watchers henceforth will have only the scraps of supposition to divine how the election is proceeding.

Emergence of two seemingly equal camps

As the conclave began, Ratzinger was considered the standard-bearer of a conservative bloc facing liberals spearheaded by Italy's Carlo Maria Martini.

Cardinal Dionigi Tettamanzi, the archbishop of Milan, and also seen as less conservative than Ratzinger, has often been cited as well.

But with the emergence of two apparently equal camps, bookmakers have also listed the likes of Nigeria's Francis Arinze, Jean-Marie Lustiger of France, Brazil's Claudio Hummes and the Honduran Oscar Andres Rodriguez Maradiaga as possible compromise candidates.

Whoever is elected pope will take the Church into a new era after 26 years under John Paul II and at a time when Roman Catholicism faces stern doctrinal and moral dilemmas.

Conservatives want to uphold his strict stance on issues such as abortion, contraception, gays and the ordination of women, but many in the Church have urged a relaxation of certain strictures, notably to allow condoms in the fight against Aids.

Issuing a clarion call for "clear faith", Ratzinger urged his fellow clerics to be guided by "the concern to bring to everyone the gift of faith and friendship with Christ".