Pope Francis. Photo/AFP

Braving the rain, tens of thousands of Catholic faithful awaited the arrival of Pope Francis in Fatima on Friday as the Portuguese holy site marks 100 years since the Virgin Mary reportedly appeared to child shepherds.

Under a menacing sky pierced by the occasional ray of sunshine, hymns echoed across the sprawling white esplanade that faces the Sanctuary of Our Lady of Fatima as pilgrims from all over the world poured in -- some seeking solace from a difficult past.

Among them was Dung Lu, a refugee from Vietnam living in Denmark after she and her mother fled their country in the late 1980s, partly because they were unable to practice their religion under the communist regime.

With tears in her eyes, she remembers praying to the Virgin Mary when she was still in Vietnam and her father was jailed after the end of the war in 1975 and then left as a boat person, leaving them behind.

"Miracles happened," she said -- her father was released from prison and survived his initial, difficult years abroad as a refugee.

Standing nearby, her young daughter interrupted her, asking why she was crying.

"It's difficult to tell them how hard it was. They are born in freedom," she said.

'Miracle' boy

Up to a million people are expected to visit this Portuguese town on Friday, heeding the pope's call to join him "as pilgrims of hope and peace."

"May your hands in prayer continue to support mine," he tweeted Friday.

Fatima has become a major holy site since the Virgin is said to have appeared six times between May and October 1917 to three impoverished, barely-literate children -- Jacinta, 7, Francisco, 9, and their cousin Lucia, 10.

She apparently shared three major prophecies with them at a time marked by the ravages of World War I and persecution of the Church in a relatively new Portuguese republic.

These reportedly included a warning of a second war and the rise of communist Russia.

On Saturday -- the 100th anniversary of the first reported apparition -- Pope Francis will canonise Jacinta and Francisco, who have officially been credited with two miracles.

One of these apparently took place in 2013, when a five-year-old Brazilian boy called Lucas recovered at lightning speed after he fell more than six metres (20 feet) from a window, smashing his skull.

His parents prayed to the late Jacinta and Francisco for help.

"The doctors, including non-believers, weren't able to explain this recovery," his father Joao Batista told reporters in Fatima on Thursday.

Rain and smiles

Many pilgrims have trekked for days on foot to the central Portuguese town -- some finishing the last few metres on their knees.

Others have staked out a prime spot next to the railings marking off the route where Pope Francis will ride his "Popemobile" later on Friday.

Carolina Palacios, a 48-year-old civil servant from Paraguay, said she and her fellow group members had taken turns sitting on fold-up chairs through the night to make sure no one took their spot.

"We came and went. We stood guard just like soldiers," she joked.

"We already saw the pope in Paraguay when he came. But it's different here.

"The weather too is different, it rains, then the sun comes out, then it rains again," she said, laughing.

Tight security

Pope Francis leaves Rome at 2:00 pm (1200 GMT) and is due to land more than two hours later at the Monte Real military base, north of Lisbon.

After a welcome by Portugal's President Marcelo Rebelo de Sousa, he will travel to Fatima by helicopter.

There, he will travel in his Popemobile to the giant esplanade that faces the Sanctuary of Our Lady of Fatima where he will be welcomed by some 400,000 faithful.

Countless others will follow proceedings on giant screens and television sets.

The day's events -- and Saturday's canonisation -- take place under high security.

Some 6,000 members of security and emergency services have been mobilised.

Cars will be banned from the immediate vicinity of the sanctuary, the airspace above will be closed and measures for jamming electronic signals implemented to prevent any flights of drones.

Border controls, meanwhile, have been reinstated in the EU country through the weekend.