Catholics from around the world gathered in Rome on Sunday for a mass presided by Pope Francis to confer sainthood on John Paul II and John XXIII -- two influential popes who helped shape 20th century history.
A million pilgrims were expected and thousands could be seen camped out in and around the Vatican, sleeping in folding chairs, praying in silence with rosary beads and singing religious chants.
Polish faithful danced around the fountains of Piazza Navona and pilgrims attended vigils across the Eternal City ahead of a ceremony seen as a way of uniting conservative and reformist strands of Catholicism.
"This is a special moment in history. The skies will open and it is going to be an extraordinary day!" said Sister Lucia Vittoria, a Brazilian living in Italy who camped out in the streets along with nine fellow nuns.
Francis will co-celebrate the open-air ceremony from 0800 GMT with pope emeritus Benedict XVI and hundreds of bishops and cardinals gathered near the altar, under giant tapestry portraits of the two late pontiffs.
This is the first time that a ruling pope and a living former pontiff celebrate mass jointly and the first time that two popes are canonised on the same day.
Francis has reached levels of popularity not seen since the glory days of John Paul II but is more comparable to John XXIII, a down-to-earth unifying figure.
By sainting them together, Francis "is speaking not just to the outside world but to rival camps within the Catholic fold who see John XXIII and John Paul II as their heroes", said US-based Vatican expert John Allen.
The ceremony will be relayed in different languages on giant screens around the city including at Fiumicino airport, the Colosseum and Piazza Navona, a square built on the site of the stadium of Domitian.
It is also being shown in 3D at cinemas across the world from Francis's native Argentina to Lebanon and live tweeted under the Vatican hashtag #2popesaints.
"It will be a great feast for our parish and especially for us Catholics who will all be united," Joseph Vinecker, a German priest at a night-time prayer vigil who said he was looking forward to "a mythical day".
Elizavet Kendzha, a Polish pilgrim, hailed John Paul II's role in helping topple Communism in her country and many of the flags around her were for Solidarity, the trade union protest movement he helped foster.
"John Paul II gave the Polish people freedom!" said the 54-year-old, who wore a red raincoat as storm clouds gathered over the Vatican after a sunny day on Saturday.
The Vatican said 98 foreign delegations would attend including former Polish president and Solidarity leader Lech Walesa and the kings of Belgium and Spain.
Zimbabwean President Robert Mugabe is also expected -- despite a ban from entering the European Union, which does not apply in the sovereign Vatican state.
There were 10,000 law enforcement on patrol and the civil protection agency has 3,500 volunteers on hand, including 25 Polish cultural guides and 20 psychiatrists trained in dealing with panic attacks.
The Vatican also launched a smartphone app called Santo Subito! (Sainthood Now) -- the phrase chanted by the crowd at John Paul II's funeral -- with practical tips for pilgrims and famous quotes from the two pontiffs.
The Vatican's official bureau for pilgrims said 4,000 coaches bearing pilgrims were arriving, along with special trains bringing the faithful from around Europe.
A Polish marathon runner jogged 2,000 kilometres (1,243 miles) to Rome in six weeks and a German vintage car collector drove in on a 1958 Polish-made car that once belonged to Karol Wojtyla before he became John Paul II.
The Polish pope was hugely popular and the first pope to be a media superstar but he also drew criticism for his clampdown on leftist movements in the Catholic Church and his failure to tackle child sex abuse.
As a Vatican diplomat, Angelo Roncalli -- the future John XXIII -- has been credited with saving thousands of Jews during World War II and he ended age-old Catholic prejudices against Jews after being elected pontiff.
But he is best remembered as the driving force behind the Second Vatican Council, which overhauled arcane Church rituals and doctrines including allowing mass to be celebrated in languages other than Latin.