He encouraged more open relations with the world, and closer ties with Judaism.

The Polish-born John Paul II - the first non-Italian pope since the Renaissance and the first from eastern Europe - reigned for nearly 27 years, from 1978 to 2005.

He was hugely popular, eschewed the pomp that surrounded his predecessors and sought contact with ordinary people. But he was criticised for failing to tackle the scourge of child sex abuse by priests. 

Delegations from 54 countries are expected, including 24 heads of state, as well as hundreds of thousands of people from across the world, who will be able to follow the ceremonies in different languages on 19 giant screens in some of the Italian capital’s most picturesque spots.

Churches will remain open all night on Saturday for prayer vigils before Sunday’s mass in St Peter’s Square honours the two Roman Catholic leaders, whose pontificates spanned from the height of the Cold War with the Cuban missile crisis to the fall of the Berlin wall.

Tapestry portraits of the new saints will hang high above the crowds and thousands of bishops, priests, and scarlet-cloaked cardinals.

A piece of John XXIII’s skin, taken when his body was exhumed for his beatification in 2000, will be on show, alongside a vial of John Paul II’s blood.

The ceremony will be attended by Costa Rican Floribeth Mora, whose claim to have been healed from a brain aneurysm by praying for John Paul’s intercession was accepted as one of the two “confirmed” miracles normally required for sainthood.

The pontiff was credited with performing another miracle just six months after his death, when a French nun said she had been cured, through prayer, of Parkinson’s – a debilitating disease from which he had also suffered.

Francis approved the canonisation of John XXIII – with whom he shares a kindly demeanour and reformist views – based on just one supposed miraculous healing, saying that the late pope was so widely adored that he did not need a second one.

The unparallelled double ceremony is seen by Vatican watchers as an attempt to bridge a traditional left-right divide in the Church, but has drawn criticism from some who argue the canonisation process was rushed.

Though becoming a saint is usually an expensive and lengthy process that can take decades, John Paul II’s elevation has been the fastest since the 18th century when the current canonisation rules were installed.

Polish Karol Wojtyla was 58 - a robust sportsman and relative outsider, who spent his holidays hiking, skiing or kayacking - when he was elected pope in October 1978. He died, aged 84, in April 2005.

An immensely popular pontiff with huge charisma, he is credited with helping bring down Communism in eastern Europe. He was nicknamed the “Pilgrim Pope” for his globe-trotting, which took him to 129 countries on flights covering more than 1.2 million kilometres.

His first foreign visit was to his native Poland; despite Soviet warnings, Communist authorities were unable to head off the pope's 1979 trip, when he appeared before million-strong crowds speaking powerfully for human rights.

It led to a huge, reinvigorated anti-communist working-class movement, the birth of the Solidarity trade union, and the steady thaw of the Soviet glacier that lay over central and eastern Europe.

He survived an assassination attempt in 1981, and his bloodstained clothing has been venerated by thousands in the run-up to the canonisation.

A right-wing Turkish extremist, Mehmet Ali Agca, shot him twice at close range as he was riding in his open-topped "popemobile" in St Peter's Square. The motives behind the assassination bid were never made clear,  but the conspiracy theories included one suggesting a KGB-ordered hit.

Despite his huge appeal, John Paul II alienated many left-wing Catholics with his conservative stance on things such as family values, homosexuality, birth control, euthanasia and abortion – particularly in Latin America.

Among those he frustrated were reformers, young people, and Third World congregations in the grip of the devastating Aids epidemic where there was disappointment over his refusal to give ground on the issue of condom use. 

He was also blamed for hushing up child sex crimes that began to come to light during his pontificate. Dogged by a rising wave of scandals of paedophile priests, the pope, at the behest of US bishops, approved new measures to punish clergymen committing  sexual abuses but only after a long silence.

He is accused of having protected “Legion of Christ” founder Father Marcial Maciel, a Mexican sexual predator and drug addict, who abused male seminarians as well as the children he fathered with different women.

Vatican spokesman Federico Lombardi has strenuously denied John Paul II knew about the abuse and has brushed off critics, saying “being a saint does not mean having done everything perfectly all one’s life”.

Asked about the crackdown on the leftist Liberation Theology movement, which was seen by conservatives as being “Marxist”, Lombardi admitted that “some debatable decisions” were taken during John Paul II’s reign.

Fellow saint-to-be “Good Pope John”, born Angelo Roncalli, made his name by calling the Second Vatican Council (1962-1965), a global gathering of Catholic bishops, less than three months after being elected. This overhauled the Church’s rituals and doctrines and reached out to other faiths.

"I wish to open the Church's window so that we may see what is happening outside and so the world may see what is happening within," the pope wrote.

Unnerving Vatican conservatives, he reached out in a famous address to crowds in St Peter's Square at the start of the Council, which spoke of his desire to bridge the gap between the Church and the faithful. "All the world is represented here tonight, even the moon hastens close to watch this spectacle. When you head home, hug and kiss your children and tell them: 'This is the hug and kiss of the pope'."

Earlier, when John was the Vatican envoy to Turkey, he was credited with saving thousands of Jews during the second world war.

He has been compared by some Vatican watchers with the current reform-minded Pope Francis.

"There is a spiritual and ideological continuity between John XXIII and Francis," said Angelo Pansa, a historian specialising in John XXIII's reign.

Time magazine named John XXIII "Man of the Year" in 1962, hailing him as a  peacemaker after his address during the Cuban missile crisis helped defuse tensions.

He died on June 3, 1963 of complications linked to stomach cancer, less than two months after he wrote the papal encyclical, Pacem in Terris (Peace on Earth). It was addressed "to all men of good will" and not only Catholics, and was in part a reaction to the prevailing political situation in the midst of the Cold War.

Pansa said: "He wanted to leave a door open to the Soviet Union, seen in the West as the Empire of Evil."

The historian said that John XXIII donated rosaries to the children of Soviet leader Nikita Khrushchev and US president John F. Kennedy - a  gesture "to unite the White House and the Kremlin".

On a more personal level, John XXIII was noted for his healthy sense of humour. Once asked by a reporter how many people worked in the Vatican, he replied: "About half."

Devotees are expected to flock to see his body in the Vatican, where he lies in a glass coffin.

Access for the ceremony itself will begin only in the early hours of Sunday, with crowds of faithful – many of them from Poland – expected to arrive by plane, train and boat overnight, before settling down with sleeping bags on the square to ensure a good view.

About 4,000 coaches carrying pilgrims are expected to arrive in the run-up to the 10am mass, in St Peter’s Square, while other faithful will watch the canonisation in 3D at cinemas around the world, from Argentina to the United States.

Sunday’s ceremony will bring the number of canonised popes to 80, out of a possible pool of 264 deceased pontiffs.

Sainthood was once relatively easy to achieve for popes during the first millennium, with all of the first 35 popes being made saints.

However, more recent pontiffs have not had the same opportunity; in the past 700 years, only two popes have become saints – Pius V in 1712 and Pius X in 1954.