By Crispian Balmer and Philip Pullella
VATICAN CITY (Reuters) - Roman Catholic cardinals prayed on Tuesday for divine help in choosing a new pope, hours before they go into a conclave to elect a pontiff who will face one of the most difficult periods in the Church's history.
The red-hatted cardinals filed into St. Peter's Basilica as choirs sang at a solemn Mass that traditionally precedes the secret conclave, which could last for several days.
Italian Angelo Sodano, dean of the cardinals, called for unity in the Church, which has been riven with intrigue and scandal, and urged everyone to work with the next pope.
"My brothers, let us pray that the Lord will grant us a pontiff who will embrace this noble mission with a generous heart," Sodano said in his homily, receiving warm applause when he thanked "the beloved and venerable" Benedict XVI.
Pope Benedict abdicated last month, saying he was not strong enough at 85 to confront the woes of a Church whose 1.2 billion members look to Rome for leadership. He has secluded himself from public life and was not present at Tuesday's service.
The Mass was the last event for the cardinals before they enter the Sistine Chapel and start their balloting for the next pontiff underneath the gaze of the divine presence represented through Michelangelo's famous fresco of the Last Judgment.
Only the 115 "princes of the Church" who are aged under 80 will take part in the vote, which is steeped in ritual. A two-thirds majority is needed to elect the new pope.
No clear favorite has emerged to take the helm of the Church, with some prelates calling for a strong manager to control the much criticized Vatican bureaucracy, while others want a powerful pastor to combat growing secularism.
Italy's Angelo Scola and Brazil's Odilo Scherer are spoken of as possible frontrunners. The former would return the papacy to Italy after 35 years in the hands of Poland's John Paul II and the German Benedict; the latter would be the first non-European pope since Syrian-born Gregory III in the 8th century.
However, a host of other candidates from numerous nations 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.
The cardinals will only emerge from their seclusion once they have chosen the 266th pontiff in the 2,000-year history of the Church, which is beset by sex abuse scandals, bureaucratic infighting, financial difficulties and the rise of secularism.
Many Catholics are looking to see positive changes.
"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 in Rome. "Every day there seem to be more."
In the past month, Britain's only cardinal elector excused himself from the conclave and apologized for sexual misconduct.
Mexican Cardinal Norberto Rivera Carrera told Italy's La Stampa newspaper there were differing views about who should be the next pontiff, with some wanting an academic, others seeking someone close to the people, and others a good manager.
Asked if the conclave could therefore drag on, he said: "I do not think it will be long ... we will come to an agreement very quickly".
The average length of the last nine conclaves was just over three days and none went on for more than five.
Signaling the divisions among Catholic ranks, Italian newspapers reported on Tuesday an open clash between prelates in a pre-conclave meeting on Monday.
The newspapers said the Vatican hierarchy's number two under Benedict, Tarcisio Bertone, had accused Brazil's Joao Braz de Aviz of leaking critical comments to the media.
Aviz retorted to loud applause that the leaks were coming from the Curia -- the Vatican's central administration which has been criticized for failing to prevent a string of mishaps during Benedict's troubled, eight-year reign.
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, might be best placed to understand the Byzantine politics of the Vatican administration - of which he has not been a part - and be able to 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 later leading Brazil's Sao Paolo diocese - the largest in a country with the biggest national Catholic community.
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.
The cardinals are expected to hold their first vote late on Tuesday afternoon - which is almost certain to be inconclusive - before retiring to a Vatican guesthouse for the night.
They hold four ballots a day from Wednesday until one man has won a two-thirds majority - or 77 votes. Black smoke from a makeshift chimney on the roof of the Sistine Chapel will signify no one has been elected, while white smoke and the pealing of the bells of St. Peter's Basilica will announce the arrival of a new pontiff.
As in medieval times, the cardinals will be banned from communicating with the outside world. The Vatican has taken high-tech measures to ensure secrecy in the 21st century, including electronic jamming devices to prevent eavesdropping.
(Additional reporting by Naomi O'Leary and Tom Heneghan; Editing by Barry Moody and Alastair Macdonald)