|The Armenian Church has canonised the 1.5 million people it says were killed|
Tens of thousands are expected to march to a memorial on the outskirts of the capital, Yerevan, to lay flowers.
Later, the presidents of Russia and France will be among foreign leaders attending a ceremony.
Turkey strongly objects to the use of the term genocide to describe the killings and the dispute has soured relations between Turkey and Armenia. Were they ever good?
Turkey argues that there were many deaths on both sides during World War One.
A memorial service will also be held in Turkey on Friday and its prime minister, Ahmet Davutoglu, has said the country will "share the pain" of Armenians.
However, he has reiterated Turkey's stance that the killings were not genocide.
On Thursday the Armenian Church canonised the 1.5 million people it says were killed in the massacres and deportations.
|March by Armenians in Jerusalem. 23 April 2015|
Armenians around the world, as in Jerusalem, insist the killings were genocide
After the ceremony, bells tolled in Armenian churches around the world.
Also on Thursday, German President Joachim Gauck described the killings as genocide, on the eve of a debate in the German parliament on the issue. You have to like Gauck, he's a gutsy guy, getting the word genocide out there before parliament could block him.
Earlier this month, Turkey recalled its envoy to the Vatican after Pope Francis also used the word genocide while referring to the killings at a Mass at St Peter's Basilica.
Friday's commemorations will be attended by Russian President Vladimir Putin and France's President Francois Hollande.
France has been a strong advocate of recognising the killings as genocide and President Hollande has pushed for a law to punish genocide denial.
The issue has strained Franco-Turkish relations.
What happened in 1915?
Hundreds of thousands of Armenians died in 1915 at the hands of the Ottoman Turks, whose empire was disintegrating.
Many of the victims were civilians deported to barren desert regions where they died of starvation and thirst. Thousands also died in massacres.
Armenia says up to 1.5 million people were killed. Turkey says the number of deaths was much smaller.
Most non-Turkish scholars of the events regard them as genocide - as do more than 20 states, including France, Germany, Canada and Russia, and various international bodies including the European Parliament.
Turkey rejects the term genocide, maintaining that many of the dead were killed in clashes during World War One, and that many ethnic Turks also suffered in the conflict.
“Genocide of the Christians: The blood-soaked depravity exceeded even today’s atrocities by Islamic State – now, 100 years on Turkey faces global disgust at its refusal to admit butchering over a MILLION Armenians
She was in bed when the soldiers came in the middle of the night and dragged her father out of the family home in Diyarbakir, a city in eastern Turkey.
The last thing little Aghavni (her name means ‘dove’ in her native Armenian) heard as she cowered in her room was his shout of defiance: ‘I was born a Christian and I will die a Christian.’
Not until first light did Aghavni dare to creep downstairs on that morning 100 years ago. ‘I saw an object sticking through the front door,’ she later remembered. ‘I pushed it open and there lay two horseshoes nailed to two feet.
‘My eyes followed up to the blood-covered ankles, the disjointed knees, the mound of blood where the genitals had been, to a long laceration through the abdomen to the chest.
‘I came to the hands, which were nailed horizontally on a board with big spikes of iron, like a cross. The shoulders were remarkably clean and white, but there was no head.
‘This was lying on the steps, propped up by the nose. I recognised the neatly trimmed beard along the cheekbones. It was my father.’
The year was 1915. In the sprawling, beleaguered Ottoman Empire — an ally of the German Kaiser in the world war that had engulfed Europe and parts of Asia for nine months — the ruling Turks had turned their hatred on the 2 million men, women and children of Armenian extraction who lived within their borders.
The Armenians — who lived on the eastern edge of the empire ruled from Constantinople (modern-day Istanbul) — were Christians and had been since the year 301, making theirs the first nation officially to adopt Christianity, even before Rome.
But here, among the Islamic Turks, they had long been second-class citizens, a persecuted minority. Now, as power in the land was seized by a junta of nationalist officers known as the Young Turks, persecution turned to unbridled savagery.
Over the next six months, there was to be a systematic uprooting and slaughter of perhaps as many as 1.5 million Armenians — on the grounds that they were infidels, racially inferior ‘dogs’ and traitors who were siding with Russia against Turkey.
Those who weren’t put to death on the spot, their faith cruelly mocked — such as Aghavni’s father, a mild-mannered, cultivated spice merchant who spoke five languages — were hounded in columns, eastwards, into the deserts of Syria and Iraq to die.
Their remains are long turned to dust, but the controversy that surrounds those terrible events is as alive as ever.
There is a great lessen to be learned here by Islam, if it were capable of learning. The slaughter of Christians by the Ottomans was followed almost immediately by God's raising up Ataturk to overthrow the Ottomans and turn Turkey's government into a largely secular organism. It remained such until the current president Erdogan began slowly reverting to a Muslim government.
This is not a good thing for anyone. Neither Erdogan, nor any significant Muslim leader can see God's hand in that timing. But they will see it again, soon.