Catholic faithful in different languages from the window of his apartment high above the crowd in St Peter's Square.
The prayer begins at 1100 GMT and usually lasts only a few minutes.
City authorities have announced tight security in and around the Vatican, with more than 100 police officers and snipers on surrounding buildings, as well as two field hospitals and hundreds of volunteers to help pilgrims.
The security is being seen as preparation for the pope's final general audience in St Peter's on Wednesday. City officials are expecting more than 100,000 people on Sunday and around 200,000 people on Wednesday.
The leader of the world's 1.2 billion Catholics has said he will step down on Thursday because he no longer has the strength of mind and body to carry on.
Last week's announcement shocked the world and brings Benedict's pontificate to an abrupt end after eight years dominated by the scandal of abuses by priests and his efforts to counter rising secularism in the West.
Benedict will be only the second pope to resign of his own free will in the Church's 2,000-year history, and the first to do so since the Middle Ages.
The momentous decision has set off a rumour mill, with some Italian media speculating his health may be far worse than the Vatican revealed and others saying an explosive report into the "Vatileaks" scandal may be to blame.
The Vatican's Secretariat of State -- effectively the government of the Catholic Church -- took the unusual step on Saturday of issuing a formal statement condemning "completely false news stories".
The Panorama news weekly and the Repubblica daily said a report by a committee of cardinals into the leaks of confidential papal papers last year had uncovered allegations of intrigue, corruption and blackmail in the Vatican.
Following Benedict's resignation, Cardinal Tarcisio Bertone who occupies a post known as the "Camerlengo Cardinal" (Chamberlain Cardinal) will take over interim powers of managing the Church before a new pontiff is elected.
Catholics around the world have been divided over the resignation. Some criticised it as a break with tradition while others welcomed it as a way of breathing new life into the Church.
No clear favourite has emerged to succeed Joseph Ratzinger. But many observers say the cardinals, who make the choice, may plump for a much younger candidate who is a more pastoral figure than the academic Benedict.
A series of meetings of cardinals starting Friday will determine the date of the start of the conclave to elect a new pope. The Vatican has hinted that it could be brought forward to early March since there is no papal funeral.
Conclaves can last for days before a candidate wins a two-thirds majority.
The Vatican has said Benedict will retire to the papal summer residence of Castel Gandolfo near Rome for the next two or three months while a former monastery inside the Vatican is renovated.
Once he moves to the monastery on a hilltop over St Peter's, the Catholic Church will be in an unprecedented and delicate situation of having a pope and his predecessor living virtually side by side within the Vatican walls.
Benedict has said he will live "hidden from the world" but Vatican spokesman Federico Lombardi has said he could provide "spiritual guidance" to his successor and will likely continue to publish his theological research.
Britain's Observer newspaper meanwhile reported Sunday allegations of inappropriate behaviour against the country's most senior Roman Catholic cleric.
It said Cardinal Keith O'Brien, leader of the Catholic Church in Scotland, has been reported to the Vatican by three priests and a former priest. O'Brien, who will be among those voting for the new pope, has contested the allegations.