Tens of thousands of people filled St Peter's Square on Sunday for a historic day of four popes, with Francis and Benedict XVI honouring their predecessors John XXIII and John Paul II and declaring them saints in the first ever canonization of two pontiffs.

Polish pilgrims carrying the red and white flags of John Paul's beloved homeland were among the first to press into the square well before sunrise, held back by human chains of neon-vested civil protection workers trying to maintain order.

Most of those who arrived first had camped out overnight on air mattresses and sleeping pads along the side streets leading to the square. Others hadn't slept at all and took part in the all-night prayer vigils hosted at a dozen churches in downtown Rome.

"Four popes in one ceremony is a fantastic thing to see and to be at, because it is history being written in our sight," marveled one of the visiting Poles, David Halfar. "It is wonderful to be a part in this and to live all of this."

The Vatican on Saturday ended weeks of speculation and confirmed that retired Pope Benedict, 87, would indeed participate in the canonization. The move sets a remarkable precedent for the 2,000-year-old Catholic Church, which has never seen a reigning and retired pope celebrating a public Mass together.

Benedict had promised to remain "hidden from the world" after resigning last year, but Francis has coaxed him out of retirement and urged him to take part in the public life of the church.

Sunday's canonization is the first time two popes have been declared saints at the same time. Francis' decision to canonize two of the 20th century's greatest spiritual leaders amounts to a delicate balancing act, giving both the conservative and progressive wings of the church a new saint.

John, who reigned from 1958-1963, is a hero to liberal Catholics for having convened the Second Vatican Council. The meetings brought the church into the modern era by allowing Mass to be celebrated in local languages rather than Latin and by encouraging greater dialogue with people of other faiths, particularly Jews.

During his quarter-century papacy from 1978-2005, John Paul helped topple communism in Poland through his support of the Solidarity movement. His globe-trotting papacy and launch of the wildly popular World Youth Days invigorated a new generation of Catholics, while his defense of core church teaching heartened conservatives after the turbulent 1960s.

1. THE CEREMONY

The preliminary part of the ceremony begins at around 0700 GMT with prayers, hymns and culminates with the chanting of the hypnotic Litany of Saints, the roll call of the church's saints, each one followed by the refrain "Ora pro nobis," or "Pray for us."

After Pope Francis and concelebrants process to the altar, the canonization rite begins immediately.

The head of the Vatican's saint-making office, Cardinal Angelo Amato, asks the pope three separate times to include John XXIII and John Paul II among the saints. In the rite for beatification, there is only one such petition. The three repeated requests for canonization "signify the importance of this celebration," noted the Vatican spokesman the Rev. Federico Lombardi.

2. THE PRONOUNCEMENT

Pope Francis then says: "For the honor of the Blessed Trinity, the exaltation of the Catholic faith and the increase of the Christian life, by the authority of our Lord Jesus Christ, and of the holy Apostles Peter and Paul, and our own, after due deliberation and frequent prayer for divine assistance, and having sought the counsel of many of our brother bishops, we declare and define Blessed John XXIII and John Paul II be saints and we enroll them among the saints, decreeing that they are to be venerated as such by the whole church. In the name of the Holy Father, and of the Son, and of the Holy Spirit."

3. THE RELICS

Relics of the two new saints are then brought to the altar. In the case of John Paul, the same relic — his blood — used for his 2011 beatification is being used. For John XXIII, a small piece of skin taken after his body was exhumed for his 2000 beatification is being used. Relics — the physical remains of a new saint or things he or she touched in life — are used to help the faithful venerate.

Amato then thanks Francis and asks him to draw up an official document attesting to the canonization. Francis responds "We so decree" and the rite ends with the singing of the "Gloria."

The Mass then proceeds as usual.

4. BY THE NUMBERS

—The Vatican said Saturday that Francis would preside over the Mass and Benedict would concelebrate along with 150 cardinals and 700 bishops. It's the first time Benedict has joined Francis in celebrating Mass in public since his resignation in 2013. Benedict attended the February ceremony in St. Peter's Basilica in which Francis installed 19 new cardinals, but as a spectator.

—About 600 priests will distribute Communion in St. Peter's Square and 210 deacons will distribute Communion to the throngs of people expected to line up along Via della Conciliazione, the main boulevard leading away from the square.

—93 official delegations attending, including an estimated 24 heads of state. The kings and queens of Belgium and Spain are expected, as are royals from Andorra, Britain and Luxemburg. Poland is sending one of the largest delegations with the current president and two former presidents, including Lech Walesa, founder of the Solidarity movement that toppled communism in Poland, which John Paul supported.

—20 Jewish leaders from the U.S., Israel, Argentina, Poland and Rome.

5. VIRTUAL CANONIZATION

—Official website www.2popesaints.org with information also available at www.vatican.va

—Vatican liturgical booklet with step-by-step process of canonization rite and order of Mass at http://www.vatican.va/news_services/liturgy/libretti/2014/20140427-libretto-canonizzazione.pdf

—Official hashtag #2popesaints