Thousands of the faithful will flock into St. Peter's Square Wednesday to bid farewell to Pope Benedict XVI at his first public appearance since the shock announcement of his resignation.
The 85-year-old Benedict, who said on Monday that he would step down on February 28 because of his advancing age, will hold a general audience from his window overlooking the square at 0930 GMT.
He will then celebrate an Ash Wednesday mass in St. Peter's Basilica at 1400 GMT, his last public mass and one of his last engagements as pope.
The mass is traditionally held in the Santa Sabina Church on Rome's Aventine Hill.
On this occasion however it has been moved, out of respect for the outgoing pontiff and to accommodate the crowd of faithful who will want to mark the end of his eight-year rule -- one of the shortest in the Church's modern history.
"It will be an important concelebration, and the last led by the Holy Father in San Pietro," Vatican spokesman Federico Lombardi said.
Key cardinals were expected to attend as the papal conclave begins gathering in Rome to elect the next head of the world's 1.2 billion Catholics.
No date has yet been set for the secret conclave, which will meet in the Sistine Chapel under Michelangelo's famous ceiling frescoes, but it could come within days of Benedict's resignation.
Lombardi has said he expects a new pope in place in time for Easter, which falls on March 31 this year. Secretary of State Tarcisio Bertone will govern the Church during the transition.
In the meantime, the outgoing pontiff will honour his existing engagements.
On Thursday he will hold his annual meeting with the pastors of Rome. And before he steps down at the end of the month he will also meet the presidents of Guatemala and Romania, as scheduled.
Candidates for the Vatican's top job will join bishops, monks, friars and pilgrims for the Ash Wednesday celebrations, during which the pope anoints the foreheads of the faithful with ashes. The ceremony launches a period of penitence before Easter in the Christian calendar.
Benedict's decision to step down -- making him the first pontiff in 700 years to resign simply because he cannot carry on -- sparked a flurry of rumours over his health, fed by revelations that he had had an operation to replace the batteries in his pacemaker three months ago.
Some observers saw Benedict's decision as a bid to avoid the fate of his predecessor John Paul II, whose drawn-out and debilitating illness was played out on the world's stage.
But Lombardi insisted: "The Pope is well and his soul is serene.
"He did not resign the pontificate because he is ill but because of the fragility that comes with old age."
After Ash Wednesday Benedict will hold another general audience and recite the Angelus twice from his apartment window on St. Peter's Square before retiring to a secluded monastery, the Vatican added.
Soon a new pope will be installed in the papal apartments, with his predecessor just a stone's throw way in the monastery which sits within the Vatican grounds.
But Benedict will spend his time in prayer rather than giving advice, says the Vatican.
Many observers say his decision to resign was "revolutionary" and argue that could change the face of the modern papacy by setting a precedent for ageing pontiffs in the future.
As rumours fly over front-runners for St. Peter's chair, commentators have said age may be a key factor in selecting a new pope, although any of the 117 cardinals eligible to vote could be chosen.
While some hope Africa or Asia could yield the next pontiff, others have tipped high-flying European or North American cardinals. The new pope will have to face up to the growing secularism in the West, one of the Church's biggest challenges.
Only one other pope has resigned because of an inability to carry on -- Celestine V in 1294 -- a humble hermit who stepped down after just a few months saying he could no longer bear the intrigue of Rome.