Retired pontiff Benedict XVI will help Pope Francis celebrate the sainthood ceremony Sunday for John Paul II and John XXIII, setting the stage for an unprecedented occurrence of two living popes canonizing two of their predecessors.

Vatican spokesman the Rev. Federico Lombardi told reporters on Saturday that Benedict will be in St. Peter's Square for the canonization of John and John Paul. He said Benedict and many cardinals will "concelebrate" the Mass with Francis.

Benedict resigned from the papacy a year ago, and since has largely dedicated himself to prayer in a monastery on the Vatican grounds. Sunday's appearance will be his highest-profile one since he retired. Francis, who lives elsewhere in Vatican City, in a guesthouse, has been quite welcoming to his predecessor, occasionally paying a call on Benedict. It was Francis who sought to include Benedict in Sunday's ceremony.

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Retired pontiff Benedict XVI will help Pope Francis celebrate the double sainthood ceremony.

"Emeritus Pope Benedict XVI has accepted the invitation, and has let Pope Francis know that he will be present tomorrow morning at the canonization ceremony and will concelebrate" along with other prelates, Lombardi said.

"That doesn't mean that he will go up on the altar" on the steps of St. Peter's Basilica, Lombardi said of the outdoor Mass. He noted that during the ceremony, cardinals and bishops will be seated on one side of the esplanade, with, presumably, Benedict, among them.

"We will all be happy to have his presence" at the ceremony, the Vatican spokesman said.

Minister of Veteran Affairs Julian Fantino is leading Canada's official delegation to the ceremony.

"I am tremendously proud and humbled to represent Canada at this historic celebration to honour the life of Pope John Paul II and Pope John XXIII," he said in a statement.

"On the eve of their canonization to sainthood, we remember their service to humanity, especially Pope John Paul II who advanced the cause of freedom and tolerance, and Pope John XXIII for his commitment to the renewal and reform of the faith."

Holiday atmosphere in Rome

St. Peter's Square was packed ahead of Sunday's canonization of two of the 20th century's great popes but hustling tour guides and a hint of the fairground were reminders that it will be a show as well as a solemn ceremony.

Pope Francis

Pope Francis will preside over the double canonization of two of modern-day Catholicism's most influential figures. (Alessandro Bianchi /Reuters)

More than a million people are expected in and around the square on Sunday.

Groups of pilgrims from Poland to Paraguay have come to attend the ceremony, drawn partly by the appeal of Francis, who has injected fresh enthusiasm into many Catholics since his appointment just over a year ago. The fame of the two popes being canonized has also attracted the faithful.

"I feel very happy because both of them inspire us," said Father Emmanuel Emeka, a priest from Nigeria. "I think both of them have a lot to teach us."

As has been true since Christian pilgrims began arriving in Rome more than 1,500 years ago, religious devotion exists side by side with the excitement of a holiday despite gloomy weather.

Tour guides in coloured T-shirts offered bus tours of the city and promised to get groups into Saint Peter's Basilica without the need for long queuing. Beggars circled wide-eyed tourists from as far away as Africa or Japan.

Alfredo Chiarelli, one of a small group of street merchants known as "urtisti," Italian for people who stop you on the street, has been busy selling religious souvenirs.

He engages curious tourists who stop to look at his tray of crucifixes, medallions and special rose-scented rosaries, his biggest seller.

"It's Pope Francis who has done this, he's really got through to people," he said, adding that the surge of interest in Francis had helped make up for pressure from cheaper Chinese-made wares and tighter spending in the economic downturn.

No sign of security problems: mayor

So far, most people have been interested in images of the Argentinian-born pope and articles connected with John Paul II, whose death brought huge crowds to Saint Peter's chanting "Santo subito!", calling for him to be made a saint immediately.

John XXIII, sometimes known as the "Good Pope" due to his friendly, humorous character, is less sought after by casual passers-by. "Most young people have no idea who John XXIII was anymore," Chiarelli says, sounding slightly regretful.

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VATICAN CITY, VATICAN - APRIL 25: Pilgrims from Costa Rica gather outside St Peter’s basilica on April 25, 2014 in Vatican City, Vatican. The late Pope John Paul II and Pope John XXIII will be canonised on Sunday 27 April, inside the Vatican when 800,000 pilgrims from around the world are expected to attend. (Dan Kitwood/Getty Images)

For those unable to pack into Saint Peter's on Sunday, 18 huge television screens will be set up around Rome, with reinforced police and paramedical teams deployed around the city to handle crowds boosted by around a million foreign visitors.

"I always knew that our pope was a saint. Now it's only going to be official," said Piotr Kurylo, a Polish endurance runner who ran all the way to Rome to attend the ceremony. "Everything he did in his life always made him a saint."

For thousands of others, special trains and planes have been laid on and many people are planning to spend Saturday night in churches that will remain open all night, praying rather than sleeping. For more earthly needs, authorities have installed 980 chemical lavatories and plan to distribute 4 million bottles of water over the weekend to keep the crowds cool.

The local Rome daily Il Messaggero reported that many bars and restaurants in the area around Saint Peter's planned to shut their doors to avoid being overwhelmed in the crush. With most of the city's hotels booked out, police were cracking down on improvised bed and breakfasts set up specially for the occasion.

The mayor of Rome, Ignazio Marino, said there were no signs of any security problems.

With files from The Associated Press, CBC News