Pope Benedict XVI will meet cardinals from around the world on Thursday in his final hours as leader of 1.2 billion Catholics before he becomes the first pontiff to resign since the Middle Ages.
The 85-year-old pontiff stunned the globe when he announced his momentous decision in a surprise speech in Latin on February 11, saying he no longer had the "strength of mind and body" to carry on in a fast-changing modern world.
"I took this step in full awareness of its gravity and novelty but with profound serenity of spirit," the pope told a cheering crowd of 150,000 pilgrims in St Peter's Square in his final public farewell on Wednesday.
The theologian pope - a shy academic whose papacy has been overshadowed by infighting within the Roman Catholic Church and a toxic sex abuse scandal - said his eight-year pontificate had seen "sunny days" and "stormy waters" but added: "I never felt alone".
The Vatican has said that the moment the pope's powers officially expire at 1900 GMT, the ex-pontiff will officially be known by the new title of "Roman Pontiff Emeritus" although he will still be addressed as "Your Holiness".
He will also keep his papal name of "Benedict XVI" and will not be referred by his original name as Cardinal Joseph Ratzinger.
Once he takes up residence as planned in a former convent on a hilltop within the Vatican walls, the Church will be in the unprecedented situation of having a pope and his predecessor living within a stone's throw of each other.
Vatican analysts have even suggested his sudden exit could set a precedent for ageing popes in the future and many Catholics say a more youthful, pastoral figure could breathe new life into a Church struggling on many levels.
But Australia's most senior Catholic, Cardinal George Pell, argued that the resignation could set a worrying precedent.
"People who, for example, might disagree with a future pope, will mount a campaign to get him to resign," Pell suggested, in comments to Australian television.
Pell, who will help choose the new pope, also criticised the outgoing pope, saying the departing pontiff was a "brilliant teacher" but "government wasn't his strong point".
From Catholic reformers calling for women clergy and for an end to priestly celibacy, to growing secularism in the West and the ongoing scourge of decades of sexual abuse by paedophile priests, the next pope will have a tough agenda.
The run-up to the exact moment that will go down in history as only the second voluntary resignation of a Roman pontiff in the Church's 2,000 years has been filled with emotion but perhaps surprisingly low-key for the Vatican.
There will be a small parting ceremony with some of his staff in a Vatican courtyard at 1550 GMT and a few minutes later the pope will board a white helicopter emblazoned with the papal insignia from the Vatican grounds.
The soon-to-be former pope will see the Vatican City - the world's smallest state - from the sky one last time as its sovereign ruler and fly to the 17th-century papal residence of Castel Gandolfo on a rocky outcrop near Rome.
There the pope will begin a quiet life of prayer and academic research.
Within a couple of months, the pope is expected to return to the Vatican and take up residence in an ex-nunnery with breathtaking views of Rome surrounded by extensive well-manicured gardens.
Benedict has said he will live "hidden from the world" but the Vatican has said he is ready to help and could provide "spiritual guidance" to the next pope although he could not intervene directly or contradict him in public.
"I am not abandoning the cross," Benedict said on Wednesday - a response to Stanislaw Dziwisz, secretary to his popular predecessor John Paul II, who said his mentor's agonising final years showed "you don't come down from the cross".
At a meeting with hundreds of tearful priests of Rome - the pope's diocese - Benedict spoke off-script about his experiences as a young reformer during the Second Vatican Council in the 1960s which changed the face of Catholicism.
"I will always be with you," he said, as they chanted "Long live the pope!"
At his last traditional weekly Angelus prayer on Sunday, he told pilgrims: "The Lord is calling me to climb the mountain."
The Vatican's culture minister Gianfranco Ravasi even compared Benedict to the Biblical figure of Moses praying to God on Mount Sinai.
Some of the pope's parting words have been harsh, such as his condemnation of "religious hypocrisy" and "rivalry" in the Church at his final mass in St Peter's Basilica where his exit was accompanied by waves of applause.
"The face of the Church... is at times disfigured," he said, following months of intrigue over a hugely damaging leak of confidential papal papers and cloak-and-dagger goings-on at the Vatican's secretive bank.