Rome (CNN) -- The work to choose a successor to retired Benedict XVI begins in earnest Tuesday, as the cardinals charged with the task prepare to be locked away in a secret election, or conclave, in Vatican City.
One of their number will almost certainly emerge from the process as the new spiritual leader of the world's 1.2 billion Roman Catholics.
Just a few hours after moving into Santa Marta, their residence at the Vatican for the duration of the process, the 115 cardinals who will choose the new pope took part in a morning Mass at St. Peter's Basilica.
They processed into St. Peter's, clad in scarlet robes, for a service in which they prayed for guidance in making a choice that could be crucial to the future direction of a church rocked by scandal in recent years.
Members of the public waited in long lines Tuesday morning to join the Mass, which was open to all. As the service began, the morning's brilliant sunshine came to an abrupt end, with the skies letting loose a torrential downpour.
Applause echoed round St. Peter's as Cardinal Angelo Sodano, dean of the College of Cardinals, offered thanks for the "brilliant pontificate" of Benedict XVI, whose shock resignation precipitated the selection of a new pope.
Sodano's homily focused on a message of love and unity, calling on all to cooperate with the new pontiff in the service of the church.
"My brothers, let us pray that the Lord will grant us a pontiff who will embrace this noble mission with a generous heart," he concluded.
In the afternoon, the 115 cardinal-electors -- those younger than 80 who are eligible to vote -- will go to the Pauline Chapel for further prayers.
They will then walk to the Sistine Chapel, chanting prayers as they go, to begin the secret election called the conclave.
The doors will be locked behind them and, 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 scaled 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 from 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.