Turkey has summoned the Vatican's Ankara envoy to express its unease after Pope Francis called the killings of Armenians by Ottoman Turks 100 years ago genocide.
Earlier the Pope called the slaughter of Armenians around the time of the First World War "the first genocide of the 20th century" and urged the international community to recognise it.
Turkey has long refused to call the event a genocide and has insisted that the toll has been inflated, and that those killed were victims of civil war and unrest.
The Pope, 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 and children who were "senselessly" murdered by Ottoman Turks 100 years ago this month.
"Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it," he said at the start of a Mass in the Armenian Catholic rite in St Peter's Basilica honouring the centenary.
In a subsequent message directed to all Armenians, Francis called on all heads of state and international organisations to recognise the truth of what transpired and oppose such crimes "without ceding to ambiguity or compromise".
Historians estimate that up to 1.5 million Armenians were killed by Ottoman Turks around the time of the First World War, an event widely viewed by scholars as the first genocide of the 20th century.
Turkey, however, has insisted that the toll has been inflated, and that those killed were victims of civil war and unrest, not genocide. It has fiercely lobbied to prevent countries, including the Holy See, from officially recognising the Armenian massacre as genocide.
Turkey's embassy to the Holy See cancelled a planned news conference today, presumably after learning that the Pope would utter the word "genocide" over its objections.
However the head of the Armenian Apostolic Church, Aram I, thanked Francis for his clear condemnation and recall that "genocide" is a crime against humanity that requires reparation.
"International law spells out clearly that condemnation, recognition and reparation of a genocide are closely interconnected," Aram said in English at the end of the Mass to applause from the pews.
Speaking as if he were at a political rally, Aram said the Armenian cause is a cause of justice, and that justice is a gift of God. "Therefore, the violation of justice is a sin against God," he said.