Huge crowds have gathered in St Peter's Square in the Vatican City for an unprecedented ceremony in which popes John Paul I and John XXIII were made saints.
It is the first time two popes have been made saints at the same time, in what experts believe is an attempt to unite the conservative and reformist wings of the Catholic Church.
A Mass will be celebrated by Pope Francis and his predecessor Benedict XVI, and watched by one million pilgrims and a huge global audience.
Heads of state and royal dignitaries were amongst the 100 foreign delegations attending.
Special bus, train and boat services transported pilgrims into Rome, for the two-hour ceremony.
Some slept overnight in St Peter's Square to be able to witness the ceremony first hand, others viewed it on giant screens erected in nearby streets.
In Rome, there was a celebratory atmosphere, as pilgrims attended special services in churches throughout Rome and played music in the city's squares.
"We've been counting down the days. This is a once-in-a-lifetime experience," said one pilgrim from Poland, John Paul II's home country.
"We're already hoarse from singing," he told AFP.
It was on Saturday that the Vatican confirmed that Benedict XVI, who became the first pope in 600 years to resign from office, would make a rare public appearance at the ceremony.
The process of canonisation usually takes years, but that of John Paul II, who died in 2005, has been fast-tracked.
Italian-born John XXIII, who during his 1958-63 papacy became known as The Good Pope, has been canonised quickly by Francis I.
In canonising both John XXIII, who initiated reforms of the Catholic Church, and John Paul I, who blocked them, experts believe Francis I has skilfully deflected accusations that he is taking sides with either wing.
However, there has been controversy over the canonisation of John Paul II, with some accusing him of failing to brings to justice catholic priests who abused children.
"There is irrefutable documentary evidence to show that John Paul II refused to take action that would have protected children during his 27-year papacy. Thousands of victims were abused because John Paul refused to read the reports he was receiving," Barbara Blaine, the president of the Survivors' Network of those Abused by Priests (SNAP), told the Telegraph.