Francis, who has close ties to the Armenian community from his days in Argentina, defended his pronouncement by saying it was his duty to honour the memory of the innocent men, women, children, priests and bishops who were "senselessly" murdered by Ottoman Turks.
"Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it," he said at the start of a Mass today in the Armenian Catholic rite in St Peter's Basilica honoring the centenary.
Turkey, however, refuses to call it a genocide and has insisted that the toll has been inflated, and that those killed were victims of civil war and unrest.
Turkey's embassy to the Holy See canceled a planned news conference today, presumably after learning that the pope would utter the word "genocide" over its objections.
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Requests for comment went unanswered and there was no official word today from the government in Ankara.
The Holy See, too, places great importance in its relationship with the moderate Muslim nation, especially as it demands Muslim leaders condemn the slaughter of Christians by Muslim extremists in neighboring Iraq and Syria.
But Francis' willingness to rile Ankara with his words showed once again that he is willing to take diplomatic risks for issues he cares deeply about.
He took a similar risk by inviting the Israeli and Palestinian presidents to pray together for peace at the Vatican -- a summit that was followed by the outbreak of fighting in the Gaza Strip.
Emeritus Pope Benedict XVI, whose ties with Turkey and the Muslim world were initially strained, avoided using the "g-word" during his pontificate.