(NBC News) -- The conclave to begin choosing the next pope got under way Tuesday, after Roman Catholic cardinals swore oaths of secrecy and the Sistine Chapel's doors were shut.
With their hand on a Bible, the 115 "princes of the church" pledged in Latin to never reveal details of the papal conclave - with the threat of ex-communication for anyone breaching ancient canonical law.
Outside the Vatican, there were cheers from onlookers gathered in front of giant television screens in St. Peter's Square when American Cardinal Timothy Dolan took his oath.
At 5:30 p.m. local time (12:30 p.m. ET) the Latin words "extra omnes" – everyone out – gave the signal for all the officials and assistants to leave the chapel.
"They're on their own now," said NBC News Vatican expert George Weigel, referring to the total isolation demanded by church rules.
None of the 115 will be seen or heard, nor will they have any contact with the outside world, until they have chosen a successor to Benedict XVI, who abdicated on Feb. 28.
The word "conclave" comes from the Latin meaning "with key". It is a church tradition that began in 1268 when local officials became so fed up with the lack of a decision among cardinals - they had deliberated for more than two years - that they locked them away with limited food and water to encourage a result.
When the voting finally begins Tuesday, there will be one ballot, followed by two every morning and two every afternoon until someone gets two-thirds of the votes.
Earlier, thousands of pilgrims and tourists waited in line to get inside St. Peter's Basilica for a special pre-conclave Mass with the cardinals.
The "Mass Pro Eligendo Pontifice" began at 10 a.m. local time (5 a.m. ET) in front of a congregation of worshippers who were waiting outside in St. Peter's Square for tickets allocated on a first-come, first-served basis.
"It's in the air! You really feel it," said Lois Girten, 55, from Austin, Texas, who secured a last-minute place on a two-week pilgrimage to Rome through a cancellation.
"It's God's gift that I'm in Rome just as the conclave takes place. I'm almost speechless with excitement, it's a real treat for me."
Several thousand visitors were allowed in to take part in the service, according to Religion News Service correspondent Alessandro Speciale inside the basilica.
'Noble mission'
In his homily, cardinal Angelo Sodano, Dean of the College of Cardinals, told the congregation: "My brothers, let us pray that the Lord will grant us a pontiff who will embrace this noble mission with a generous heart."
At night, cardinals will walk or be taken by minibus the short distance to the modest rooms in Casa Santa Marta, which John Paul II had built in 1996.
Such is the importance of secrecy that Vatican officials have installed jamming devices to prevent the use of cellphones by cardinals or hidden microphones by anyone wanting to hear their deliberations.
Although there is no definitive favorite to take the helm, cardinals have been holding a series of General Congregations in recent days to discuss the qualities they would like to see in their new leader.
No conclave has lasted more than five days in the past century, with most finishing within two or three days. Pope Benedict was elected within barely 24 hours in 2005 after just four rounds of voting.
Benedict triggered the election last month with his unexpected decision to abdicate because of his increasingly frail health -- the first pontiff to step down in six centuries.
He leaves his successor a sea of troubles -- including seemingly never-ending sex-abuse scandals, rivalry and strife inside the Vatican bureaucracy, a shortage of priests and a rise of secularism in its European strongholds.
