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By Claudio Lavanga

YEREVAN, Armenia — A somber Pope Francis paid tribute on Saturday to the 1.5 million Armenians massacred in 1915, an event which he labeled a genocide a day earlier, risking Turkey's ire.

"Here I pray, with pain in my heart, so that never again will there be tragedies like this, so that humanity does not forget and knows how to overcome evil with good," he wrote in the guest book at the "Genocide Memorial and Museum" on Saturday.

Visibly moved, Francis participated in a prayer service along with President Serzh Sarksyan and leaders of the Armenian Apostolic Church.

Pope Francis (right), accompanied by Catholicos of All Armenians Karekin II, arrives to lead an open-air mass in Vartanants Square in Armenia's second-largest city of Gyumri on Saturday.TIZIANA FABI / AFP - Getty Images

On Friday, the pope broke a century-old taboo by using the word "genocide" hours after landing in Armenia's capital for a three-day visit to the former Soviet state.

Francis strayed from his prepared address at the presidential palace in Yerevan and referred to the slaughter of ethnic Armenians by Ottoman Turks as a "genocide," which marked "the start of a sad series of great catastrophes of the last century."

The use of the term is bound to enrage Turkey, which denies accusations of ethnic cleansing, claiming the deaths occurred in the fog of World War I and the crumbling of the Ottoman Empire.

It is not the first time Francis has described the large-scale killings as genocide. During a Mass marking the 100th anniversary of the deaths in April 2015,he said the events were “the first genocide of the 20th century,” echoing a phrase first used by Pope John Paul II in a 2001 joint declaration ahead of his own trip to Armenia.

Related: Pope Francis to Visit Armenia Amid 'Genocide' Spat With Turkey

But in Armenia this week, Francis became the first pope on Armenian soil to utter the word in such context.

Turkey did not immediately react to the pope’s comment in Yerevan, but his use of the word last year compelled the country to summon the Vatican ambassador to Ankara and recall its own ambassador to the Holy See for almost a year.

Relations between Turkey and the Vatican could reach a new low — and be exacerbated — if the pontiff decides to repeat the word “genocide” on Saturday morning. He plans to visit the Armenian genocide memorial in Yerevan and pray. On his official program, the site is referred to as the "memorial of the massacres."

Reuters contributed.