VATICAN CITY — Cardinals heard a final appeal for unity Tuesday before sequestering themselves in the Sistine Chapel for the conclave to elect the next pope, as they celebrated Mass amid divisions and uncertainty over who will lead the 1.2 billion-strong Catholic Church and tend to its many problems.

A Gregorian chant wafting through St. Peter’s Basilica, the 115 cardinal electors filed in wearing bright red vestments, many looking grim as if the burden of the imminent vote was weighing on them.  

A few hundred people braved thunderstorms and pouring rain to watch the Mass on giant TV screens in St. Peter’s Square. A handful knelt in prayer, eyes clenched and hands clasped.

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 and the next pope.

“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.

“Let us pray for the cardinals who are to elect the Roman pontiff,” read one of the prayers during the Mass. “May the Lord fill them with his Holy Spirit with understanding and good counsel, wisdom and discernment.”

One of the faithful outside alluded to the huge challenge facing the next pontiff.

“It’s a moment of crisis for the church so we have to show support of the new pope,” said Veronica Herrera, a real estate agent from Mexico who traveled to Rome for the conclave with her husband and daughter.

In the afternoon, the cardinals 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 3 p.m. EDT — black for no pope, white if a pope has been chosen.

While few people expect a pontiff to be elected on the first ballot, the Vatican was ready: In the Room of Tears off the Sistine Chapel where the pope goes immediately after his election, three sizes of white cassocks hung from a clothes rack. Underneath, seven white shoe boxes were piled, presumably containing the various sizes of the red leather shoes that popes traditionally wear. The room gets its name from the weight of the job thrust upon the new pontiff.

The papal tailor Gammarelli delivered the clothes on Monday to ensure that the newly elected pope could change immediately into papal white as soon as he accepts the election. With the words “Habemus Papam” — or “We have a pope” — the pontiff then appears on the balcony of St. Peter’s Basilica to greet the crowd for the first time.

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.

Even the American cardinals couldn’t agree on whether to expect a short or long conclave.

Dolan this week expressed optimism that the election would be wrapped up quickly. That appeared to be in stark contrast with the view of Chicago Cardinal Francis George: His spokeswoman, Colleen Dolan, told The Associated Press that the cardinal suggested it could be a drawn-out affair.

George raised the possibility that the cardinals may still be meeting by Saturday, Dolan said, when conclave rules require the cardinals to take a break and spend some time in prayer before resuming voting.

The faithful in St. Peter’s square were also weighing in on the papal stakes.

“I don’t think it’s going to be a European pope,” said Michael Flueckiger, a 38-year-old caretaker and sacristan of a church in Slamatt, Switzerland. “In Europe sometimes I think we have given away the gift of faith, many people have lost the faith, they have lost their expectation in God.”

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.