Mary is said to have appeared in the central Portuguese town of Fatima on six different occasions to three shepherd children, starting from May 13, 1917.

Two of them, Francis Marto and his sister Jacinta, aged 7 and 9 at the time, were elevated to sainthood in the mass held at the Our Lady of Fatima sanctuary in Portugal to mark the anniversary on Saturday.

The third child who saw the Madonna, Lucia de Jesus dos Santos, a cousin to the two siblings, became a nun and died in 2005, aged 97. The recognition of her sainthood is still in the works.

Francis is on the second and final day of his trip to Fatima, about 130 kilometres north of Lisbon.

On Friday, hundreds of thousands of the faithful lined the streets, chanting and singing, as the hoped to catch a glimpse of the pope as he drove by.

After praying in private at the shrine, he then led public evening prayers

The first vision in Fatima is said to have occurred as the children were pasturing sheep on the Cova da Iria field. They saw a lady dressed in white holding a rosary and claiming to have come from heaven.

Appearing in the midst of World War I, she told the children to “Pray the Rosary every day, in order to obtain peace for the world, and the end of the war,” according to memoirs Lucia wrote in the 1930s.

In a second apparition, the Madonna said she would “soon” take Jacinta and Francisco, who died in the following three years from Spanish influenza, while Lucia, who grew up to be a nun and passed away in 2005, aged 97, would remain on earth “some time longer”.

In the third apparition, the children saw a vision of hell and were told predictions known as the Three Secrets of Fatima.

As later revealed by Lucia, they included the outbreak of World War II, and the “reconversion” of Communist Russia to Christianity.

Lucia refused to reveal a third secret at the time but informed the Vatican about it.

Pope John Paul II disclosed it in 2000, saying it prophesied his 1981 assassination attempt.