The Mass was attended by Pope Emeritus Benedict, who quit as pope last year, and roughly 100 foreign delegations.
Analysts say Francis is trying to balance the conservative legacy of John Paul with the reforming zeal of John.
The ancient rite of canonisation unfolded under grey skies in a packed St Peter’s Square. In keeping with tradition, Pope Francis, was approached and requested to add his two predecessors to the long list of Catholic saints. And at the third time of asking, he granted that request.
Then ornate, silver containers holding holy relics of new saints were shown: a trace of blood from John Paul II, and sliver of skin taken from the body of John XXlll. Both men were hugely influential figures in the story of modern Catholicism.
The Italian Pontiff, John XXlll, is seen very much as a liberal, reforming figure. The Polish Pope, John Paul, on the other hand, was much more conservative. And their elevation to the sainthood on the same day is being seen as an attempt to draw together the liberal and the more traditional wings of the Church.
At the climax of the service, Pope Francis said in Latin: “We declare and define Blessed John XXIII and John Paul II to be saints and we enrol them among the saints, decreeing that they are to be venerated as such by the whole Church.”
Relics of each man – a container of blood from John Paul and a piece of skin from John – were placed near the altar.
Pope Francis paid tribute to the two new saints as “priests, bishops and popes of the 20th Century”.
“They lived through the tragic events of that century, but they were not overwhelmed by them. For them, God was more powerful,” he said.
The Vatican estimated some 800,000 pilgrims had poured into Rome to see the two-hour ceremony first-hand.
For those unable to make it into St Peter’s Square, giant screens were set up in nearby streets and elsewhere in the city.
“Four popes in one ceremony is a fantastic thing to see and to be at, because it is history being written in our sight,” said Polish pilgrim Dawid Halfar.