The ceremony, which is believed to become the biggest canonisation service in history, comes ahead of commemorations expected to see millions of people including heads of state tomorrow mark 100 years since the start of the killings.
The Armenian Apostolic Church announced the canonisation service for the "martyrs of the Armenian Genocide", calling for a "prayerful participation in this historic event".
The service will be held in Armenia's main church, Echmiadzin, an austere fourth-century edifice believed to be the Christian world's oldest cathedral.
"Today's canonisation unites all Armenians living around the globe," Huri Avetikian, an ethnic Armenian librarian from Lebanon who arrived in her ancestral homeland to attend the service, told AFP.
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After the ceremony to be led by Catholicos of All Armenians, Karekin II, bells will chime in Armenian churches across the world and a minute of silence will be observed.
"The Armenian Church will proclaim the collective martyrdom of those who were killed over their faith and their homeland," church spokesman Father Ter Vahran told AFP.
Ex-Soviet Armenia and the huge Armenian diaspora worldwide have battled for decades to get the World War I massacres at the hands of the Ottoman forces between 1915 and 1917 recognised as a targeted genocide.
But modern Turkey - the successor to the Ottoman Empire - has refused to do so, and relations remain frozen to this day.
In a rare interview with Turkish television broadcast today, Armenian President Serzh Sarkisian expressed hope that the two countries could mend fences.
"It is obvious that a reconciliation between the two peoples will have to come about through Turkey recognising the genocide," he told CNN-Turk.