Tens of thousands were expected to attend Pope Benedict XVI's last Sunday prayers, as cardinals began arriving in Rome to elect his successor and the Vatican battled reports of high intrigue.
The 85-year-old pope will read out his traditional Angelus prayer and messages to 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 today 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 yesterday 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.