UPDATE: Black smoke rose Tuesday from the chimney over the Sistine Chapel, indicating that a new pope has not been selected.
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ROME (CNN) -- The heavy wooden doors to the Sistine Chapel swung closed Tuesday, signaling the start of the secret election, or conclave, in which 115 Roman Catholic cardinals will pick the next pope.
Now all eyes will turn to the chimney installed on the roof of the historic chapel.
From this point on, the only clue the world will have of what is happening inside will be periodic puffs of smoke that follow each round of voting.
Black smoke, no pope. White smoke, success.
On a day rich with symbolism, the scarlet-clad cardinals entered the Sistine Chapel in solemn procession, chanting prayers and watched over by the magnificent paintings of Renaissance artist Michelangelo
Each of the cardinal-electors -- those under age 80 who are eligible to vote -- then swore an oath of secrecy, led by Cardinal Giovanni Battista Re, the most senior cardinal in the conclave.
A designated official then gave the order in Latin, "Extra omnes" -- that is, "Those who are extra, leave."
With all those not taking part in the conclave gone, the cardinals will remain locked in total isolation until one candidate can garner two-thirds of their votes.
That man will emerge from the process as the new spiritual leader of the world's 1.2 billion Roman Catholics.
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.
Earlier, the cardinals celebrated a special morning Mass at St. Peter's Basilica, where 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 thunder, lightning and a torrential downpour.
Applause echoed around St. Peter's as Cardinal Angelo Sodano, dean of the College of Cardinals, offered thanks for the "brilliant pontificate" of Benedict XVI, whose unexpected 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.
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.
The cardinals moved Tuesday morning into Santa Marta, their residence at the Vatican for the duration of the conclave.
Jamming devices have been put in place to stop the cardinal-electors from communicating with the outside world using mobile phones or other electronic means.
When cardinals elected Benedict in 2005, the white smoke signaling the decision came about six hours after an earlier, inconclusive vote, Lombardi 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 be chosen?
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.
According to CNN Vatican analyst John Allen, also a correspondent for the National Catholic Reporter, the race is wide open as the cardinals enter the conclave.