Pope Francis is to make two shepherd children saints at the Fatima shrine complex in Portugal 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 on Friday and later greeted pilgrims in Fatima. More than a million are expected.
Portugal has boosted security and re-imposed border controls temporarily.
Roman Catholic pilgrims have converged on the Fatima Sanctuary from countries as far away as China, Venezuela and East Timor.
Plea for harmony
On Friday, Pope Francis flew into Fatima, north of Lisbon, in a helicopter and travelled through town in his “Popemobile”.
At a candle-lit vigil he called for harmony between all people at the Chapel of the Apparitions and spoke of wars “tearing our world apart”.
The chapel is built on the very spot where the Virgin Mary is said to have appeared.
The Pope will leave Fatima soon after Saturday’s mass, ending a 24-hour trip.