Rome (CNN) -- The Catholic cardinals gathered in Rome voted Friday to begin the secret election, or conclave, to elect a new pope next Tuesday afternoon, the Vatican said.

The 115 cardinal-electors taking part in the conclave will enter the closed-door process after a morning Mass, the Vatican said. Only those younger than 80 are eligible to vote.

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The cardinals voted Friday morning to accept the letters of explanation of two cardinal-electors who are eligible to vote for the next pope but will not attend the conclave: Keith O'Brien of Scotland and Julius Riyadi Darmaatmadja of Indonesia.

Darmaatmadja cited health reasons, and O'Brien cited personal reasons.

O'Brien resigned in scandal last week after allegations that he made sexual advances toward young men studying to be priests. He apologized in a statement Sunday, saying, "There have been times that my sexual conduct has fallen below the standards expected of me as a priest, archbishop and cardinal."

Since Monday, the cardinals have been uniting for what are known as General Congregations, a series of meetings in which they discuss the issues facing the church.

These include how to tackle the issue of child sex abuse by priests and a scandal over leaks from the Vatican last year that revealed claims of corruption within the Roman Catholic Church hierarchy.

The chimney used to send up the smoke signals that announce whether or not a new pope has been elected could be raised over the Sistine Chapel on Friday, said the Rev. Federico Lombardi, a Vatican spokesman.

The voting takes place inside the chapel, beneath the ornate ceiling painted by Michelangelo.

The building was closed to tourists Tuesday and will remain so for the foreseeable future, the Vatican said. Work is well under way to transform it for the conclave.

The cardinals have sworn an oath of secrecy. Nonetheless, the Vatican is taking no chances.

An electronic shield will be put up around the conclave to prevent the use of mobile phones and other devices that might allow communication with the outside world.

However, Lombardi denied a report in the Italian media that the cardinals will be searched as they go in and out of the conclave.

Among the things the cardinals have to do is draw lots for which rooms they get at the Casa Santa Marta, the hotel within the walls of Vatican City where they stay during the conclave.

The hotel also contains a suite where the newly elected pope will live for a few weeks before moving to the papal apartments.

The apartments were placed under seal after Benedict XVI left last week and must be renovated before the new pope can take up residence, the Vatican said.

Conclave to Elect New Pope Set for Tuesday