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John XXIII, who reigned from 1958 to 1963 and set up the modernising Second Vatican Council, and John Paul II, the Pole who reigned for nearly 27 years and played a leading role on the world stage, will be declared saints by Pope Francis.
Francis' own huge popularity has added extra appeal to the unprecedented ceremony to raise two former leaders of the church to sainthood. But while both were widely revered, there has also been criticism that John Paul II, who only died nine years ago, has been canonised too quickly.
Groups representing victims of sexual abuse by Catholic priests also say he did not do enough to root out a scandal that emerged towards the end of his pontificate and which has hung over the church ever since.
The controversy has however done nothing to put off the hundreds of thousands of Catholic faithful who have been arriving in Rome over the past few days.
"Pope John Paul II was the pope who opened the doors to the youth and was very close to us young people," said Argentinian nun, Sister Irmana Mariella.
"I grew up with him and it is very emotional for me, especially to share and to transmit all this, personally, to all the people in Argentina who want to be here but cannot."
Some 10,000 police and security personnel and special paramedical teams were to be deployed and large areas of Rome closed to traffic, in an operation mayor Ignazio Marino estimated would cost more than 7 million euros.
Thousands of pilgrims from Poland were due to arrive in specially chartered trains at around midnight on Saturday, with many expected to spend the night praying in Rome churches left open especially for the event.
Despite occasional bursts of drenching rain, thousands of others gathered in groups dancing or singing in the centre of Rome or settled in for an overnight wait near Saint Peter's in the hope of securing a spot when the square is opened early on Sunday morning.
"It rains a lot at home so we don't have any real worries about waiting to be in the square tomorrow," said Emmanuel Dalieux, a pilgrim from the southern French city of Lourdes. "We're not afraid of the rain at all."
The appointment of the Argentinian-born Pope Francis has injected fresh enthusiasm into a church beset by sexual and financial scandals during the papacy of his predecessor Benedict XVI, who resigned last year in a step not seen since the Middle Ages.
He now lives in secluded retirement but will be present at the canonisation mass, which is due to begin at 10 a.m. (0800 GMT) and which will bring together four popes at one time.
The fact that the two being canonised are widely seen as representing contrasting faces of the church has added to the political significance of an event that Francis hopes will draw the world's 1.2 billion Catholics closer together.
John, an Italian often known as the "Good Pope" because of his friendly, open personality, died before the Second Vatican Council ended its work in 1965 but his initiative set off one of the greatest upheavals in church teaching in modern times.
The Council ended the use of Latin at mass, brought in the use of modern music and opened the way for challenges to Vatican authority which alienated some traditionalists.
John Paul was widely credited with helping to bring down communist rule in eastern Europe and hastening the end of the Cold War. He continued many of the reforms but tightened central control, condemned theological renegades and preached a stricter line on social issues such as sexual freedom.
A charismatic, dominant pope, he was criticised by some as a rigid conservative but the adoration he inspired was shown by the huge crowds whose chants of "santo subito!" (make him a saint at once!) at his funeral 2005 were answered with the fastest declaration of sainthood in modern history.
Both canonisations have involved some intervention with the normally strict rules governing declaration of a saint, which involve a close examination of each candidate's life and works and normally the attestation of at least two miracles.
Benedict waived a rule that normally requires a five-year waiting period before the preliminaries to sainthood can even begin to speed up John Paul's canonisation while Francis ruled that only one miracle was needed to declare John a saint.