There was no official crowd estimate, but the main square has a capacity of 600,000 and was overflowing. Authorities said they expected 1 million people.
"It is a big emotion because it is the first time he comes here, and we are always hopeful that something can improve," said Laurindo Pereira, a 55-year-old carpenter. "It is faith. In everything, it is about faith and it is a beautiful thing."
Francis is spending fewer than 24 hours in Fatima to celebrate the centenary of the apparitions and canonize two of the three shepherd children. He is hoping the message of peace that they reported 100 years ago, when Europe was in the throes of World War I, will resonate with the Catholic faithful today.
During an evening vigil at the chapel built on the site of the apparitions, Francis bowed his head in silent prayer before the statue of the Virgin and left a gold-leaf rose at its base, following a tradition of popes leaving golden roses at Marian shrines.
In a prayer, he urged the faithful to follow in the footsteps of the young shepherds and spread peace even in times of war.
"We will tear down all walls and cross every frontier, as we go out to every periphery, to make known God's justice and peace," he said.
The Fatima mystery has fascinated Catholics and non-Catholics alike for a century, blending visions of the Virgin, supernatural meteorological events and apocalyptic messages of hell, World War II, communism and the death of a pope.
It all began on May 13, 1917, when three cousins, aged 7-10, Francisco and Jacinta Marto and Lucia dos Santos, reported that they had seen a vision of the Virgin Mary as they grazed their sheep. They returned to the same spot in the coming months and reported similar visions.
Portuguese church officials initially doubted them. Many doubters, though, became believers after the so-called "miracle of the sun" on Oct. 13, 1917. The children had predicted that the Virgin would perform a miracle that day, and tens of thousands of people flocked to Fatima and saw what witnesses reported was a vision of the sun "spinning" in the sky and zigzagging toward Earth.
The Portuguese church declared the visions authentic in 1930.
The children reported that the Madonna revealed to them three "secrets," and urged them to pray for peace and turn away from sin. Francisco and Jacinta Marto died of influenza two years later, and will be declared saints by the pope Saturday, the 100th anniversary of the initial vision.
Lucia, who later became a nun and wrote of the children's experiences, is on track for beatification, the first step toward becoming a saint. Her case couldn't begin until after her death in 2005.
Francis' deputy, Cardinal Pietro Parolin, has said the importance of Fatima lies in the fact that poor, illiterate children — not the wealthy, the learned or intellectuals — were able to convey a powerful message of love and forgiveness at a time of war, when "the talk was of hatred, vendetta, hostilities."
Fatima has long been associated with St. John Paul II, given that the Polish-born pope credited the Virgin Mary with saving his life in 1981 when a would-be assassin shot him on Fatima's feast day — May 13 — in St. Peter's Square. John Paul made the first of three pilgrimages to Fatima the following May, and one of the bullets fired at him now adorns the crown of the Madonna statue at the shrine.
In 2000, the Vatican disclosed the long-awaited third secret of Fatima, describing it as foretelling the 1981 assassination attempt and the persecution of other Christian faithful.
Like John Paul, the Argentine-born Francis is exceedingly devoted to the Madonna, thanks in large part to the strong role that Marian devotions play among Latin American Catholics.