Francis laid a wreath at the memorial and stood, head bowed, in silent prayer before an eternal flame as priests blessed him with incense and a choir sang haunting hymns.
"Here I pray with sorrow in my heart so that a tragedy like this never again occurs, so that humanity will never forget and will know how to defeat evil with good," Francis wrote in the memorial's guest book. "May God protect the memory of the Armenian people. Memory should never be watered-down or forgotten. Memory is the source of peace and the future."
After visiting the Tzitzernakaberd memorial, Francis headed to the northwestern city of Gyumri, where crowds filled a main square for his only public Catholic Mass of the three-day trip to Armenia. He was ending his day with a prayer for peace back in the capital Yerevan, for what the Vatican said would be the largest gathering of his visit.
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In the most carefully watched speech of the trip, Francis on yesterday ad-libbed the politically charged word "genocide" to his prepared text, listing the Armenian genocide alongside the Holocaust and Stalinism as the three great mass slaughters of the 20th century.
There was no immediate reaction from Turkey, which withdrew its ambassador last year and accused Francis of spreading lies when he first termed the slaughter a genocide.
"Sadly that tragedy, that genocide, was the first of the deplorable series of catastrophes of the past century, made possible by twisted racial, ideological or religious aims that darkened the minds of the tormentors even to the point of planning the annihilation of entire peoples," Francis said.
"It's so sad how, in this case and in the other two, the great international powers looked the other way," he added, referring to the subsequent horrors of Nazism and Stalinism.