Turkey has recalled its envoy to the Vatican after Pope Francis described the mass killing of Armenians under Ottoman rule in WW1 as "genocide".
Turkey has reacted with anger to the comment made by the Pope at a service in Rome earlier on Sunday.
Armenia and many historians say up to 1.5 million Armenian Christians were killed by Ottoman forces in 1915.
But Turkey has always disputed that figure and said the deaths were part of a civil conflict triggered by WW1.
The row has continued to sour relations between Armenia and Turkey.
The Pope made the comments at a Mass in the Armenian Catholic rite at Peter's Basilica, attended by the Armenian president and church leaders.
He said that humanity had lived through "three massive and unprecedented tragedies" in the last century.
"The first, which is widely considered 'the first genocide of the 20th Century', struck your own Armenian people," he said, in a form of words used by a declaration by Pope John Paul II in 2001.
Pope Francis also referred to the crimes "perpetrated by Nazism and Stalinism" and said other genocides had followed in Cambodia, Rwanda, Burundi and Bosnia.
He said it was his duty to honour the memories of those who were killed.
"Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it," the Pope added.
Armenia's President Serzh Sargsyan welcomed his comments, saying they sent a powerful message to the international community.
But Turkey immediately summoned the Vatican's ambassador to Ankara for an explanation, and then later recalled its ambassador from Rome.
The foreign ministry said it felt "great disappointment and sadness" at the Pope's remarks, which it said would cause a "problem of trust" between them.
Turkey's Foreign Minister Mevlut Cavusoglu tweeted: "The Pope's statement, which is far from the legal and historical reality, cannot be accepted.
"Religious authorities are not the places to incite resentment and hatred with baseless allegations," he added.
Analysis: David Willey, BBC News, Rome
Pope Francis, who visited Turkey last year, would have been perfectly conscious that he would offend the moderate Muslim country by his use of the word "genocide".
But the Pope's powerful phrase "concealing or denying evil is like allowing a wound to bleed without bandaging it" extended his condemnation to all other, more recent, mass killings.
It now remains to be seen how far his remarks will impact upon the Vatican's future relations with moderate Muslim states. It was a bold decision but totally coherent with Pope Francis' philosophy of open discussion about moral arguments.
Pope Francis' focus today on Armenia, the first country to adopt Christianity as its state religion, even before the conversion of the Roman Emperor Constantine, serves as yet another reminder of the Catholic Church's widely spread roots in Eastern Europe and the Middle East.
In 2014, for the first time, Turkish PM Recep Tayyip Erdogan offered condolences to the grandchildren of all the Armenians who lost their lives.
But he also said that it was inadmissible for Armenia to turn the issue "into a matter of political conflict".
Armenia says up to 1.5 million people died in 1915-16 as the Ottoman Empire was disintegrating. Turkey has said the number of deaths was much smaller.
Many of the victims were civilians deported en masse to barren desert regions where they died of starvation and thirst. Thousands also died in massacres.
Most non-Turkish scholars of the events regard them as genocide. Among the other states which formally recognise them as genocide are Argentina, Belgium, Canada, France, Italy, Russia and Uruguay.
Turkey maintains that many of the dead were killed in clashes during World War One, and that ethnic Turks also suffered in the conflict.