VATICAN CITY (AP) - Cardinals celebrated a final Mass on Tuesday before sequestering themselves in the Sistine Chapel for the conclave to elect the next pope, seeking to overcome their divisions and rally behind a single man who can lead the 1.2 billion-strong Catholic Church and tend to its many problems.

As a Gregorian chant filled St. Peter's Basilica, the 115 cardinals who will participate in the conclave filed in wearing bright red vestments, many looking grim as if the burden of the imminent vote was weighing on them.

In his homily, Cardinal Angelo Sodano, dean of the College of Cardinals, called for unity within the church, a not-so-veiled appeal to the cardinal electors to put their differences aside for the good of the church.

"Each of us is therefore called to cooperate with the Successor of Peter, the visible foundation of such an ecclesial unity," Sodano said.

He was interrupted by applause from the pews - not so much from the cardinals - when he referred to the "beloved and venerated" Benedict XVI and his "brilliant" pontificate.

Benedict's surprise resignation - the first in 600 years by a pope - has thrown the church into turmoil and exposed the deep divisions among cardinals who are grappling with whether they need a manager who can clean up the Vatican's dysfunctional bureaucracy or a pastor who can inspire Catholics at a time of waning faith.

In the afternoon, the 115 cardinal electors will file into the frescoed Sistine Chapel singing the Litany of Saints, a hypnotic chant imploring the intercession of saints to help them choose a pope.

They will hear a meditation by an elderly Maltese cardinal, take an oath of secrecy, then in all probability cast their first ballots.

Assuming they vote, the first puffs of smoke should emerge from the chapel chimney by 8 p.m. (3 am Singapore time) - black for no pope, white if a pope has been chosen.

The conclave is taking place amid more upheaval and uncertainty than the church has seen in decades: There's no front-runner, no indication how long voting will last and no sense that a single man has what it takes to be pope.

The buzz swirled around Cardinal Angelo Scola, an Italian seen as favored by cardinals hoping to shake up the powerful Vatican bureaucracy, and Brazilian Cardinal Odilo Scherer, a favorite of Vatican-based insiders intent on preserving the status quo.

Other names included Canadian Cardinal Marc Oullet, who heads the Vatican's powerful office for bishops, and American Cardinal Timothy Dolan.

Going into the vote, cardinals offered wildly different assessments of what they're looking for in the next pontiff and how close they are to a decision. It was evidence that Benedict XVI's surprise resignation has continued to destabilize the church leadership and that his final appeal for unity may go unheeded, at least in the early rounds of voting.

A few cardinals also sent their last tweets before entering the conclave, which forbids communication with the outside world.

"Heavenly Father, guide our hearts and grant us wisdom and strength tomorrow," Ghanaian Cardinal Peter Turkson, considered to have an outside chance to be pope, tweeted late Monday.