Pope Francis has greeted pilgrims at the Fatima shrine complex in Portugal, where he plans to make two shepherd children saints on Saturday.
It is 100 years since the two - and a third child - reported seeing the Virgin Mary while tending sheep. The third is also on the way to sainthood.
The Pope arrived at a military airbase north of Lisbon. More than a million pilgrims are expected at Fatima.
Portugal has boosted security and reimposed border controls temporarily.
Roman Catholic pilgrims have converged on the Fatima Sanctuary from countries as far away as China, Venezuela and East Timor.
There will be a torch-lit procession on Friday and a Mass on Saturday. The Pope will leave Fatima soon after, ending a 24-hour trip.
Two of the children - Jacinta and Francisco Marto - are to be canonised on Saturday for the miracles attributed to them. They died in the 1918-1919 European influenza pandemic.
The so-called three secrets of Fatima were written down by their cousin, Lucia dos Santos, who died in 2005 aged 97. The beatification process for her began in 2008.
The Church attaches great value to their visions, as Mary is believed to have revealed truths to help mankind. The Church says the first vision came on 13 May 1917.
In a video message to the people of Portugal, the Pope said he was going to present himself to Mary "and I need to feel you close, physically and spiritually, so that we are one heart and one mind".
What are the three secrets?
They are prophecies written down by Lucia, years after the apparitions that the three said they had witnessed. She spent her adult life as a nun at a convent in Coimbra.
The first two secrets in Lucia's account were revealed in 1942.
- The first described a terrifying vision of hell, with a "great sea of fire", demons and human souls
- The second is interpreted as Mary's prediction that World War One would end and that World War Two would start during the papacy of Pius XI
- Mary also called for the "consecration" of Russia, saying: "If my requests are heeded, Russia will be converted, and there will be peace; if not, she will spread her errors throughout the world, causing wars and persecutions of the Church"
- Lucia sealed the third secret in an envelope, which was handed to the Vatican in 1957 and only revealed in 2000
- It described an angel demanding "penance!", then the Pope and other clergy climbing a mountain, only to be killed by soldiers firing bullets and arrows.
What does the Vatican say about them?
According to Pope Francis's predecessor, Benedict XVI, the visions described in the three secrets are "meant to mobilise the forces of change in the right direction".
They are not like the Bible - a text he describes as a "public revelation".
The Fatima visions are "private revelations", he writes. Their purpose is "to help live more fully" in accordance with Christ's teaching.
The late Pope John Paul II was shot by a Turkish gunman on 13 May 1981.
He believed that his survival was due to Mary's divine intervention, and that the third secret had predicted the attack on him.
John Paul donated the bullet to Fatima, and it was inserted into the crown adorning a statue of Mary there.
What about Pope Francis's visit?
He follows John Paul II and Benedict XVI, who also made pilgrimages to the Fatima Sanctuary.
Portugal is deploying 6,000 police and emergency workers at the site daily. Concrete blocks have been placed on approach roads, to stop any terrorist "ramming" attack with a vehicle.
Only nine border crossings are open, with systematic checks, as Portugal has suspended the Schengen open borders pact.
A Portuguese man called Carlos Gil is known as "rent-a-pilgrim", AFP news agency reports. He charges €2,500 (£2,110; $2,717) to walk to Fatima and worship there on behalf of a Catholic who cannot make the trip.
Local accommodation is far more expensive than usual, as hotels and residents cash in on the papal visit.