Pope John Paul II and Pope John XXIII are to be declared saints at an unprecedented open-air ceremony in Rome on Sunday.
A Mass co-celebrated by Pope Francis and his predecessor, Pope Emeritus Benedict, will be witnessed by one million pilgrims and a vast TV and radio audience.
Nearly 100 foreign delegations are due, including royal dignitaries and heads of state and government.
It is the first time two popes have been canonized at the same time.
Correspondents say the move is being seen as an attempt to unite conservative and reformist camps within the Roman Catholic Church.
Pilgrims have been pouring into Rome and special bus, train and boat services ferried many more into the city early on Sunday morning for the two-hour ceremony which starts at 10:00 local.
Some had bagged places to sleep overnight as close as possible to St Peter’s Square, hoping to be among the first in when it opens to the public.
Giant screens have also been erected in nearby streets and elsewhere in the city for those unable to get into the square.
The Vatican confirmed on Saturday that 87-year-old Pope Emeritus Benedict XVI would make a rare public appearance alongside his successor, Pope Francis.
“He will co-celebrate, which does not mean he will go to the altar,” a Vatican spokesman said.
“We will all be happy to have him there.”
Benedict XVI became the first pope to resign for 600 years when he quit for health reasons a year ago, sending shock waves around the world.
The process of saint-making is usually long and very costly.
However, John Paul II, whose 26-year reign ended in 2005, has been fast-tracked to sainthood in just nine years.
Many among the huge crowds that gathered as he lay dying cried out “Santo subito!” (Make him a saint immediately!).
By contrast Italian-born John XXIII, known as the Good Pope after his 1958-1963 papacy, had his promotion to full sainthood decided suddenly and very recently by Pope Francis.
By canonizing both John XXIII – the pope who set off the reform movement – and John Paul II – the pope who applied the brakes – Pope Francis has skillfully deflected any possible criticism that he could be taking sides.
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