VATICAN CITY — Pope Francis declared Popes John XXIII and John Paul II saints before some 800,000 people on Sunday in an unprecedented ceremony made even more historic by the presence of emeritus Pope Benedict XVI in St. Peter's Square.

Never before have a reigning pope and a retired pope celebrated Mass together in public, much less at an event honoring two of their most famous predecessors.

Benedict's presence was a reflection of the balancing act that Francis envisioned when he decided to canonize John and John Paul together, showing the unity of the Catholic Church by honoring popes beloved by conservatives and progressives alike.

Francis made that point clear in his homily, praising both new saints for their work associated with the Second Vatican Council, the groundbreaking meetings that brought the 2,000-year-old institution into modern times. John convened the council in 1962 while John Paul helped ensure its more conservative implementation and interpretation.

"John XXIII and John Paul II cooperated with the Holy Spirit in renewing and updating the church in keeping with her pristine features, those features which the saints have given her throughout the centuries," Francis said.

He praised John for having allowed himself to be led by God to call the council, and he hailed John Paul's focus on the family — an issue Francis has taken up himself.

"They were priests, bishops and popes of the 20th century," Francis said. "They lived through the tragic events of that century, but they were not overwhelmed by them."

It was Benedict who put John Paul on the fast-track for possible sainthood just weeks after he died in 2005, responding to the chants of "Santo Subito!" or "Sainthood Now!" that erupted during his funeral Mass. His canonization is now the fastest in modern times.

Francis then tweaked the Vatican's own saint-making rules, deciding that John could be made a saint alongside him without the necessary second miracle usually required for canonization.

Francis took a deep breath and paused for a moment before reciting the saint-making formula in Latin at the start of the ceremony, as if moved by the history he was about to make in canonizing two popes at once.

He said that after deliberating, consulting and praying for divine assistance "we declare and define that 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."

Applause broke out from a crowd that stretched from St. Peter's to the Tiber River and beyond.

The Vatican estimated that 800,000 people watched the Mass in Rome, with about 500,000 in the square and nearby streets and the rest watching on TV screens that had been set up in piazzas around town.