Photo: Kayana Szymczak

The soft sound of the duduk, the national instrument of Armenia, enveloped the Cathedral of the Holy Cross Saturday afternoon at the start of a prayer service held on the eve of the 101st anniversary of the Armenian genocide,  reports.

Cardinal Sean P. O’Malley led the ecumenical service that marked the first time the Archdiocese of Boston has formally commemorated the genocide that killed 1.5 million people.

“It is so important that we do not allow the events of the genocide to slip into oblivion,” O’Malley said, addressing nearly 800 people seated in the pews. “The one and a half million lives are not forgotten. . . . One of the fruits of their martyrdom is the accumulation of love that unites us.”

O’Malley was joined by church leaders representing a number of Catholic, eastern Orthodox, Christian, and Armenian churches across the eastern United States and Canada.

Sunday marks 101 years since the beginning of the mass execution by the Ottoman Empire. In a ceremony one year ago, those killed were canonized by the Armenian Apostolic Church.

“We pray for them,” said Archbishop Oshagam Choloyan, leader of the Armenian church in the eastern part of the United States. “They will be remembered for eternity.”

Pope Francis last year declared the mass killing of Armenian Christians in Ottoman Turkey during World War I “the first genocide of the 20th century,” during a Mass in St. Peter’s Square to mark the centennial anniversary of the massacre.

The remark angered officials in Turkey, which does not recognize the wartime killings as a genocide.

O’Malley, a top American adviser to the pope, on Saturday urged people to confront the lessons of the genocide.

“We must ask ourselves if the world had responded differently to the Armenian Genocide, could the Holocaust [have been] averted?” he said.

“No civilization can afford to falsify the historical record,” he said. “To do so is perilous.”

Anthony Barsamian, the cochairman of the Armenian Assembly of America, said present-day Turkey must “account for its past so that they will not repeat the crime of genocide.”

He lauded O’Malley for holding the two-hour prayer service.

“We warmly thank Cardinal Sean O’Malley and the Massachusetts Catholic community for hosting this important event on our behalf,” said Barsamian, who also serves as president of the Massachusetts Council of Churches, an ecumenical organization.

Some Armenians said they were grateful for the chance to pray with people of other faiths.

“It was a beautiful ceremony of unity,” said Sona Topjian Frissora, 87, who lives in the North End. “I can’t tell you how touched I was.”

“It was very memorable for me,” said Lilit Karapetyan of Watertown, during a reception that followed the service. “The most amazing thing is there were people from all religions there.”