Pope Francis prays at the Sanctuary of Our Lady of Fatima Friday, May 12, 2017, in Fatima, Portugal. Pope Francis will canonize on Saturday two poor, illiterate shepherd children whose visions of the Virgin Mary 100 years ago marked one of the most important events of the 20th-century Catholic Church. (AP Photo/Alessandra Tarantino)
Pope Francis prays at the Sanctuary of Our Lady of Fatima Friday, May 12, 2017, in Fatima, Portugal. Pope Francis will canonize on Saturday two poor, illiterate shepherd children whose visions of the Virgin Mary 100 years ago marked one of the most important events of the 20th-century Catholic Church. (AP Photo/Alessandra Tarantino) Credit: Alessandra Tarantino

FATIMA, Portugal — Pope Francis urged Catholics on Friday to “tear down all walls” and spread peace as he traveled to this Portuguese shrine town to mark the 100th anniversary of one of the most unique events of the 20th-century Catholic Church: the visions of the Virgin Mary reported by three shepherd children and the “secrets” she told them.

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.

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.

The Fatima mystery has fascinated Catholics and non-Catholics alike for a century, blending visions of the Virgin, supernatural weather 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 went 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.