Two young, barely literate shepherds who had visions of the Virgin Mary 100 years ago in Fatima, a Portuguese site now a global draw for pilgrims, were declared saints Saturday by Pope Francis.

In an emotional outdoor service on a packed esplanade at the Sanctuary of Our Lady of Fatima, Catholic worshippers from around the world sang and looked on, some crying, while many more watched the canonisation on giant screens from adjacent streets.

Altogether, about 500,000 people were present, the Vatican said in a statement, quoting local authorities -- far below initial expectations of 800,000 to a million.

"We declare the blissful Francisco Marto and Jacinta Marto saints," the Argentine pontiff said in front of the white basilica where the siblings are buried, with two giant portraits of the child shepherds hanging in the background.

When the two impoverished siblings first reportedly had visions of the Virgin Mary 100 years ago on the spot where the sanctuary now lies, they were probably far from imagining they would one day join the ranks of prominent saints like Mother Teresa.

The Virgin Mary is said to have appeared six times between May and October 1917 to Jacinta, then 7, Francisco, 9 and their cousin Lucia, 10.

She apparently shared three prophesies with the trio at a period marked by the ravages of World War I, which the Church believes included a vision of hell and a warning of a second major conflict.

The third secret, which was made public by the Vatican only in 2000, foretold the attempted assassination in Rome of Pope John Paul II on May 13, 1981, the anniversary of the first apparition in Fatima.

Pope Benedict XVI later gave an updated interpretation of this third prophesy, saying it could include the suffering of the Church following sexual abuse scandals that shook the Vatican.

- Two 'miracles' -

Francisco died in 1919 and Jacinta the following year in the Spanish Influenza epidemic that swept through Europe at the end of the war.

Their cousin Lucia lived until 2005, becoming a nun and meeting several popes.

A process that could lead to her also becoming a saint has been opened.

Pope John Paul II was possibly the most devoted to Fatima, attributing his narrow escape from death to the intervention of the Virgin Mary.

He beatified Jacinta and Francisco in 2000 -- the final step before sainthood.

The siblings were canonised after the Church officially attributed two miracles to the pair -- a necessary step to become a saint.

Wheelchair-bound Maria Emilia Santos said she regained the ability to walk on February 20, 1989, the anniversary of Jacinta's death, after praying to her.

And the parents of a Brazilian boy say he healed at lightning speed after falling more than six metres from a window in 2013, after they prayed to Jacinta and Francisco.

- Miracle or hallucination? -

Fatima attracts millions of worshippers every year for whom the story of apparitions and miracles adds powerful fuel to their faith.

"They're our little shepherds, it means everything to us," said Luisa Pacheco, a 48-year-old seamstress from the northern region of Porto who spent the night in her car to see the ceremony.

Yet talk of apparitions and miracles outside of those described in the Old and New Testaments does not sit comfortably with everyone in the Church.

At the time of the reported apparitions, local authorities considered the child shepherds to be troublemakers, throwing them briefly in jail before they were released under public pressure.

The alleged apparitions, seen only by the trio, were followed by strange phenomena in October 1917 when some 70,000 onlookers waiting for a sign of the Virgin are said to have seen the sun dancing in the sky.

But to this day, Catholics remain divided over whether these were truly apparitions or merely collective hallucination reinforced by a natural phenomenon.

Generally speaking, the official recognition of visions or miracles follows a long and cautious process, so as not to risk the Church's credibility.

The Vatican, for instance, has yet to recognise the reported continuous apparitions of the Virgin Mary in the Bosnian town of Medjugorje.

© Agence France-Presse