By Thomas Mackintosh, BBC News, London and Bill Heaney in Dumbarton
The closed-door meeting will start inside the Sistine Chapel and will involve some 135 cardinals from across the world.
It follows the death of Pope Francis, left, who died at the age of 88 on Easter Monday and whose funeral was held on Saturday.
There is no timescale as to how long it will take to elect the next pope, but the previous two conclaves, held in 2005 and 2013, lasted just two days.
Vatican spokesman Matteo Bruni said cardinals will take part in a solemn Mass at St Peter’s Basilica, after which those eligible to vote will gather in the Sistine Chapel for the secretive ballot.
Once they enter the Sistine Chapel, pictured at top of page, cardinals must have no communication with the outside world until a new Pope is elected.
There is only one round of voting on the first afternoon of the conclave, but the cardinals will vote up to four times every day afterwards.
A new pope requires a two-thirds majority – and that can take time.
Each cardinal casts his vote on a simple card that says, in Latin: “I elect as Supreme Pontiff” to which they add the name of their chosen candidate.
If the conclave completes its third day without reaching a decision, the cardinals may pause for a day of prayer.
Outside the Sistine Chapel the world will be watching for the smoke from the chimney.
If the smoke is black, there will be another round of voting. White smoke signals that a new pope has been chosen.
On Saturday, politicians and royalty joined thousands of mourners as Pope Francis’ funeral Mass was held in St Peter’s Square.
Hymns played out on giant speakers, occasionally drowned out by the sound of helicopters flying overhead, before 91-year-old Cardinal Giovanni Battista Re gave a homily on the pope’s legacy.
After a ceremony, huge crowds lined the streets of Rome to watch as the Pope’s coffin was carried in a procession to his final resting place, Santa Maria Maggiore Basilica.
Authorities said 140,000 people had lined the streets, clapping and waving as the hearse – a repurposed white popemobile – crossed the Tiber river and drove past some of Rome’s most recognisable sights: the Colosseum, the Forum and the Altare della Patria national monument on Piazza Venezia.
On Sunday images of Pope Francis’s tomb at the church were released showing a single white rose lying on the stone that bears the name he was known by during his pontificate, below a crucifix illuminated by a single spotlight.
Meanwhile, Angelo Becciu, an Italian cardinal convicted of embezzlement and stripped of his privileges by Pope Francis, has confirmed he will not take part in the conclave to elect a new Catholic leader.
In a statement from his lawyer, Cardinal Becciu said he would abide by the late pontiff’s will, ending days of speculation on whether he would join other cardinal electors in the Sistine Chapel on 7 May.
“I have decided to obey, as I have always done, the will of Pope Francis not to enter into conclave,” Cardinal Becciu said, while continuing to protest his innocence.
The 76-year-old said he decided to take a step back “having at heart the good of the Church” and “to contribute to the communion and serenity of the Conclave”.
The cardinal, a former adviser to Francis who was once considered a papal contender himself, was removed from office and stripped of his cardinal “rights and privileges” in September 2020.
He subsequently went on trial along with nine others in a case focused on a disastrous investment by the Vatican in a luxury building in London.
In 2023, he was sentenced to five years and six months in jail for financial crimes, although he is appealing the conviction.
Cardinal Becciu, once one of the most powerful figures in the Vatican, had reportedly been pushing to take part in the conclave, despite not being on the official list of electors.
Yesterday, Cardinal Pietro Parolin, who as secretary of state was Francis’s number two, presented him with two documents signed by the Argentine pontiff which said he could not, according to Italian media.
The letters dated from 2023 and last month, according to the Domani newspaper.
Cardinal Becciu was the most senior clergyman in the Catholic Church to face a Vatican criminal court.
His very rare punishment came as Francis enacted a series of reforms aimed at cleaning up the Vatican’s notoriously murky finances.
Cardinal was number two in the Secretariat of State from 2011 to 2018, during most of which time Cardinal Parolin was his boss.
Cardinal Parolin is considered by many to be a frontrunner for the next pope, due to his vast diplomatic experience.
But Vatican expert John L Allen Jr sounded a note of caution last week, saying questions over his role in the London real estate scandal could play against him.
- There will be no representation from Scotland at the Conclave since Cardinal Keith Patrick O’Brien, who was given a red hat by the late Saint John Paul II, is now dead. He failed to attend the Conclave which elected Pope Francis when he retired in disgrace from St Andrews and Edinburgh archdiocese after being caught up in a sex scandal involving young priests. Cardinal O’Brien’s family was from Dalmuir in West Dunbartonshire and he was a former pupil of St Patrick’s High School in Dumbarton.