ROME (CNN) -- The work to elect a successor to retired Pope Benedict XVI begins in earnest Tuesday, when the cardinals charged with the task gather in Vatican City for the papal conclave.
Just a few hours after moving into Santa Marta, their residence at the Vatican for the duration of the process, the cardinals entered a morning Mass at St. Peter's Basilica.
The service -- open to the public -- is the last public event featuring the 115 cardinals who will choose the new spiritual leader of the world's 1.2 billion Roman Catholics.
They processed ceremonially into St. Peter's, dressed in scarlet robes.
The cardinal-electors -- those aged under 80 who are eligible to vote -- will then walk to the Sistine Chapel, chanting prayers as they go, to begin the secret election called the conclave.
After that, the only clue the world will have of what is happening inside will be periodic puffs of smoke from a copper chimney installed over the weekend in the Sistine Chapel.
Black smoke, no pope. White smoke, success.
Rome is abuzz
Rome was abuzz Monday with preparations for the conclave, from the 5,600 journalists the Vatican said had been accredited to cover the event to the red curtains unfurled from the central balcony at St. Peter's, the spot where the world will meet the new pope once he is elected.
Tailors have also completed sets of clothes for the new pope to wear as soon as he is elected, in three different sizes.
Video released by the Vatican over the weekend showed the installation of a pair of stoves inside the chapel. One is used to burn the cardinals' ballots after they are cast and the other to send up the smoke signal -- the one that alerts the world that a vote has been taken and whether there's a new pope.
Workers could be seen scaling the roof of the chapel Saturday to install the chimneys.
When we'll see the first smoke is anyone's guess.
An electronic shield has been put in place to stop the cardinal-electors communicating with the outside world using mobile phones or other devices.
Cardinal Roger Mahony, the retired Archbishop of Los Angeles, tweeted early Tuesday: "Last tweet before moving to Casa Santa Martha, and Mass to Elect a Pope. First Conclave meeting late Tuesday afternoon. Prayers needed."
The cardinals will probably vote Tuesday, but they don't have to, Vatican spokesman the Rev. Federico Lombardi said Monday.
If they do, it's likely the first smoke might be seen around 8 p.m. (3 p.m. ET), he said.
When cardinals elected Benedict in 2005, the white smoke signaling the decision came about six hours after an earlier, inconclusive vote, he said.
It took another 50 minutes for Benedict to dress, pray and finally appear on the balcony of St. Peter's, he said.
The longest conclave held since the turn of the 20th century lasted five days.
On Monday, cardinals held the last of several days of meetings, known as General Congregations, to discuss church affairs and get acquainted. Lombardi said 152 cardinals were on hand for the final meeting.
Church rules prevent cardinals over the age of 80 from participating in the conclave but allow them to attend the meetings that precede the vote.
Who will win?
Meanwhile, the Italian press is full of speculation about which cardinal may win enough support from his counterparts to be elected, and what regional alliances are being formed.
"Many would say it's all about politics at this point," Monsignor Rick Hilgartner, head of U.S. Conference of Catholic Bishops Secretariat on Divine Worship, told CNN, "but I think it's important to remember that they also recognize that this is a very spiritual moment." Once the doors close and conclave begins, he says it's less about politicking and "more about prayer as they each in silence write their votes."
Italy potentially wields the most power with 28 of the 115 votes, making it the largest bloc in the College of Cardinals. The United States is second with 11. Altogether 48 countries are represented among the cardinal- electors.
Sixty-seven of their number were appointed by Benedict, who stepped down at the end of last month, becoming the first pontiff to do so in six centuries.
This article was written by CNN's Laura Smith-Spark, Richard Allen Greene and Dan Rivers. CNN's Dan Rivers and Richard Allen Greene reported from Rome, and Laura Smith-Spark wrote in London. CNN's Ed Payne, Michael Pearson and Hada Messia and journalist Livia Borghese also contributed to this report. TM & © 2012 Cable News Network, Inc., a Time Warner Company. All rights reserved.