In a procession to a hilltop memorial in the capital Yerevan, Armenians will carry candles and flowers to lay at an eternal flame, as members of the diaspora that fled as a result of the slaughter commemorate the sombre anniversary in cities thousands of miles away.
Russian President Vladimir Putin and his French counterpart Francois Hollande are expected to be among a handful of leaders to travel to Armenia for the commemorations, but others are shying away for fear of upsetting Ankara.
More than 20 nations - including France and Russia - have so far recognised the Armenian genocide, a definition supported by numerous historians.
German President Joachim Gauck was expected to draw an angry reaction from Turkey after he condemned the massacres as genocide for the first time, speaking at a religious service commemorating the bloodletting.
Gauck said the then German empire - the Ottoman Turkey's ally in WWI - bore "shared responsibility, possibly shared guilt for the genocide."
Ankara on Wednesday recalled its ambassador to Vienna in response to Austrian lawmakers' decision to condemn the massacre as "genocide".
Turkey has said up to 500,000 were killed, but mostly due to war and starvation, and rejects the use of that term.
US President Barack Obama yesterday would only go so far as to describe the World War I massacre as "terrible carnage".
Commemorations expected to draw millions in Yerevan, Paris, Los Angeles and beyond will come a day after the canonisation service, which made saints of the the 1.5 million Armenians that historians believe to have perished.
"During the dire years of the genocide of the Armenians, millions of our people were uprooted and massacred in a premeditated manner, passed through fire and sword, tasted the bitter fruits of torture and sorrow," Catholicos of All Armenians, Karekin II, said at the ceremony.
