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 said the job of pope is to be merciful, charitable and "tirelessly promote justice and peace."
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. Sitting in the front row was Benedict's longtime aide, Archbishop Georg Gaenswein.
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.
For over a week, they met behind closed doors to try to figure out who among them had the stuff to be pope and what his priorities should be. But they ended the debate on Monday with questions still unanswered and many cardinals predicting a drawn-out election that will further expose the church's divisions.
"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."
In his final radio address before being sequestered, U.S. Cardinal Timothy Dolan on Tuesday said a certain calm had taken hold over him, as if "this gentle Roman rain is a sign of the grace of the Holy Spirit coming upon us."
He said he at least felt more settled about the task at hand. "And there's a sense of resignation and conformity with God's plan. It's magnificent," he said during his regular radio show on "The Catholic Channel" on SiriusXM.
"It's almost a microcosm of life itself, you know how you try to make the right decisions in conformity with God's holy will. And I think that's what's happening now. I just hope I see you soon."
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.
Yet the mood was not entirely somber or reverent.
A group of women who say they are priests launched pink smoke from a balcony overlooking the square during the Mass to demand female ordination a play on the famous smoke signals that will tell the world whether a pope has been elected. The Femen group of women activists, several of whom have gone topless in St. Peter's to protest the Vatican's opposition to gay marriage, were also due to protest Tuesday.
And in a bizarre twist, basketball star Dennis Rodman is expected to arrive in St. Peter's Square on Wednesday in a makeshift popemobile as he campaigns for Cardinal Peter Turkson of Ghana to become the church's first black pope.
None of the cardinals will see it, since they will be sequestered inside the Vatican walls, allowed only to go from the Vatican hotel through the gardens to the Sistine Chapel and back again until they have elected a pope. No telephones, no newspapers, no television, no tweeting.
The cardinals begin this process Tuesday afternoon by filing 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. (1900 GMT; 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 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.
Cardinal Dolan this week publicly expressed optimism that the election would be wrapped up quickly. And on the eve of the conclave, he wrote a letter to New York priests, saying: "My guess is that we'd have a new Successor of St. Peter by Thursday evening," according to Dolan's spokesman, Joseph Zwilling.
That bullish stance stood 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 long affair. George raised the possibility that the cardinals may still be meeting by Saturday, 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 Flamatt, 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," Turkson, the Ghanian cardinal considered to have an outside chance to be pope, tweeted late Monday.
Rachel Zoll, Karl Ritter and Daniela Petroff contributed.
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