VATICAN CITY (Reuters) - Thick black smoke billowed into the night sky from the Sistine Chapel's chimney on Tuesday, signaling an inconclusive first vote in the conclave to elect a new pope at a time of strife and scandal for the Roman Catholic Church.

Thousands of faithful huddled in St. Peter's Square to watch the smoke pour out of the narrow flue in the rain-laden gloom following a day rich in ritual and pageantry.

After praying for divine guidance, the red-hatted cardinals took a solemn vow, in Latin, never to divulge any details of their deliberations. They then secluded themselves behind the chapel's heavy wooden doors.

No conclave in the modern era has chosen a pope on its first day, and some cardinals speculated this week that it might take four or five days to pick the man to replace Pope Benedict, who unexpectedly abdicated last month.

The so-called "Princes of the Church" will spend the night in a Vatican hotel before returning to the frescoed Sistine Chapel on Wednesday to continue their votes, with two rounds set for the morning and two for the afternoon.

Until they chose a new pontiff, their only communication with the outside world will be the smoke from the Chapel chimney - black when voting sessions end with no result and white when a pontiff is elected.

Whoever becomes the 266th pontiff in the Church's 2,000-year history will face a daunting array of problems, including sex abuse scandals, infighting within the Vatican bureaucracy and the spread of secularism in its European heartland and beyond.

No clear-cut front runner has emerged, with some prelates pushing for a strong manager to control the much criticized central administration, known as the Curia, while others want a powerful pastor to promote their faith across the globe.

Italy's Angelo Scola and Brazil's Odilo Scherer are spoken of as strong contenders. The former would return the papacy to Italy after 35 years in the hands of Poland's John Paul II and the German Benedict XVI. Scherer would be the first non-European pope since Syrian-born Gregory III in the 8th century.

However, a host of other candidates have also been mentioned as "papabili" - potential popes - including U.S. cardinals Timothy Dolan and Sean O'Malley, Canada's Marc Ouellet and Argentina's Leonardo Sandri.

EXTRA OMNES

Latin chants accompanied the cardinals as they processed into the Sistine Chapel, with Michelangelo's depiction of Christ delivering the Last Judgment on the back wall and his image of the hand of God giving life to Adam on the ceiling.

The doors were shut at 5.34 p.m. after the master of ceremonies, Guido Marini, said "Extra Omnes" (Latin for "Everyone Out"), asking all those not associated with the gathering to leave the room.

Maltese Cardinal Prosper Grech, who at 87 is too old to participate in the voting, remained inside to give a sermon to remind the 115 cardinal electors of the gravity of their responsibility.

Crowds of tourists and Italians soon gathered in the vast square to catch a glimpse of the famous smoke, including former European Commission president, Romano Prodi.

"I have just come as an ordinary citizen to see this," he told Reuters, accompanied by his family. "And isn't it beautiful."

Police detained two women who staged a brief topless protest against the Church before the massed ranks of television crews who have come from around the world to follow the conclave.

Earlier, at a pre-conclave Mass in St. Peter's Basilica, Italian cardinal Angelo Sodano called for unity in the Church and urged his brother cardinals to support the future pope.

"My brothers, let us pray that the Lord will grant us a pontiff who will embrace this noble mission with a generous heart," he said in his homily.

LOOKING BEYOND EUROPE

The faithful gathered in Rome hoped their new leader would bring change after Benedict's difficult eight-year reign.

"He must be a great pastor with a big heart, and also have the capacity to confront the Church's problems, which are very great," said Maria Dasdores Paz, a Brazilian nun who attended the Mass. "Every day there seem to be more."

In the past month, the only British cardinal elector recused himself from the conclave and apologized for sexual misconduct.

All the prelates in the Sistine Chapel were appointed by either Benedict XVI or John Paul II, and the next pontiff will almost certainly pursue their fierce defense of traditional moral teachings.

But Benedict and John Paul were criticized for failing to reform the Curia, and some churchmen believe the next pope must be a good chief executive or at least put a robust management team in place under him.

Vatican insiders say Scola, who has managed two big Italian dioceses without being part of the Vatican's central administration, could be well placed to understand its Byzantine politics and introduce swift reform.

The still-influential Curia is said by the same insiders to back Scherer, who worked in the Vatican's Congregation for Bishops for seven years before leading the Sao Paolo diocese - the largest in Brazil, the country with the most Catholics.

With only 24 percent of Catholics living in Europe, pressure is growing to choose a pontiff from elsewhere in the world who would bring a different perspective.

Latin American cardinals might worry more about poverty and the rise of evangelical churches than questions of materialism and sexual abuse that dominate in the West, while the growth of Islam is a major concern for the Church in Africa and Asia.