* Conclave starts Tuesday night with 115 cardinal electors
* Prelates will hold last pre-conclave meeting on Monday
* No frontrunner seen, possibility of longer conclave (Adds colour, quotes, details)
By Philip Pullella and Crispian Balmer
VATICAN CITY, March 10 (Reuters) - Roman Catholic Cardinals prayed on Sunday for spiritual guidance ahead of a closed-door conclave to choose a new pope to lead the Church at one of the most difficult periods in its history.
Cardinals will hold a final pre-conclave meeting on Monday to discuss the state of their Church, left reeling by the abdication last month of Pope Benedict and struggling to deal with a string of sexual abuse and corruption scandals.
The 115 cardinals who will take part in the secret ballots, which start on March 12, fanned out around Rome on Sunday to hold myriad Masses, either in the quiet of private chapels or in the grandeur of Rome's great cathedrals and basilicas.
Each cardinal is traditionally assigned to a church in the Italian capital and congregations swelled in parishes visited by those considered the most likely papal contenders -- such as Cardinal Odilo Pedro Scherer of Sao Paulo, Brazil.
"We're all preparing for the conclave because we need to make the right decision to decide who is going to be the new pope," Scherer told a small Baroque church in the heart of Rome, crammed with well-wishers.
He was later driven away in a minivan with darkened windows, declining to speak to the waiting hoards of reporters -- a taste of the pressures to come if he should become the first non-European to be elected pope in some 1,300 years.
Just up the road, another non-European touted as a possible candidate, U.S. Cardinal Sean O'Malley, also received star treatment as he arrived for Mass in ornate vestments.
"I say sincerely that we hope this is your last visit as cardinal," said parish priest father Rocco Visca, prompting loud applause and cheers from the well-heeled congregation.
A coach load of faithful from northern Italy travelled down to Rome to hear Milan's cardinal, Angelo Scola, give a sermon at the monumental Santi Apostoli church.
"Let us pray that the Holy Spirit gives the Church a man who can lead her in the footsteps of the great pontiffs of the past 150 years," said Scola, seen as the leading Italian candidate.
Like fellow cardinals, he appeared eager not to draw too much attention to himself and exited quietly via a back door.
Some cardinals, such as Manila's Luis Antonio Tagle, who is considered a long-shot because of his relatively young age, 55, kept an even lower profile, mostly staying inside the walls of seminaries or other religious institutions.
Open canvassing is frowned upon in the run-up to the conclave, with prelates aware of the Rome saying "he who enters the conclave a pope comes out a cardinal".
Vatican spokesman Father Federico Lombardi said the so-called princes of the church had been in constant contact in recent days and had reached initial conclusions.
"They therefore feel ready to confront the decisive step of electing a new pope," he told Vatican Radio.
The 115 cardinal electors under the age of 80 will enter the Sistine Chapel on Tuesday afternoon and hold one vote that evening. They will vote up to four times day thereafter until one of their number receives a two-thirds majority, or 77 votes.
If a pope is not elected in two or three days it means that cardinals are probably severely divided and might have to turn to a dark horse candidate to find consensus.
No conclave has lasted than more than five days in the past century. Pope Benedict was elected within barely 24 hours in 2005 after just four rounds of voting. But this time, no clear favourites have emerged to take the helm of the troubled Church.
Apart from Scola, Scherer and O'Malley, other potential candidates most mentioned are Canada's Marc Ouellet, U.S. cardinal Timothy Dolan and Argentina's Leonardo Sandri.
It was unclear how much the geographical distribution of the cardinals would weigh. Sixty cardinals come from Europe, including 28 Italians, while there are 19 from Latin America, 14 North Americans, 11 Africans, 10 Asians and one from Oceania.
The Italians held the papacy for 455 years before the 1978 election of Polish-born Pope John Paul.
Many of the Italian cardinals work within the Vatican bureaucracy, which has come under heavy criticism in recent years because of infighting and perceived incompetence.
Some Italian newspapers said many of the Italian prelates were rallying around Scherer, while many outsiders favoured Scola, believing he had the clout and knowledge needed to revitalise and reform the creaking Vatican government. (Additional reporting by Tom Heneghan, Anna Valderama, Elly Biles and Naomi O'Leary; Editing by Stephen Powell)