German President Joachim Gauck has described as "genocide" the killings of Armenians by Ottoman Turks, a move likely to cause outrage in Turkey.
He was speaking on the eve of a debate in the German parliament on the issue.
The Armenian Church earlier canonised 1.5 million Armenians it says were killed in massacres and deportations by Ottoman Turks during World War One.
Turkey disputes the term "genocide", arguing that there were many deaths on both sides during the conflict.
On Friday commemorations will mark the 100th anniversary of the killings.
Speaking at a church service in Berlin, President Gauck said: "The fate of the Armenians stands as exemplary in the history of mass exterminations, ethnic cleansing, deportations and yes, genocide, which marked the 20th Century in such a terrible way."
He described the 1915 atrocity as "planned and systematic mass murder".
Mr Gauck, who holds a largely ceremonial role, added that Germans also bore some responsibility "and in some cases complicity" concerning the "genocide of the Armenians". Germany was an ally of the Ottoman Empire during World War One.
He said Germany not only knew about the determination to exterminate the Armenians, but German soldiers "were also involved in the planning and, partly, in the execution of the expulsions".
His comments come as the German parliament, the Bundestag, prepares to debate a motion on the 1915 massacres.
But instead of a clear statement of condemnation, politicians will discuss an opaque, tortuously-worded sentence, which aims to be unclear enough to keep everyone happy - with the sort of convoluted phrasing that the German language is so good at, the BBC's Damien McGuinness in Berlin reports.
Earlier on Thursday, the Armenian Church said the aim of the canonisation ceremony near the capital Yerevan was to proclaim the martyrdom of those killed for their faith and homeland.
After the ceremony, bells tolled in Armenian churches around the world.
The beatification at the Echmiadzin Cathedral did not give the specific number of victims or their names.
It is the first time in 400 years that the Armenian Church has used the rite of canonisation.
The use of the word "genocide" to describe the killings is controversial. Pope Francis was rebuked recently by Turkish President Recep Tayyip Erdogan for describing it as the "first genocide of the 20th Century".
On Friday, a memorial service will be held in Turkey and its prime minister, Ahmet Davutoglu, has said the country will "share the pain" of Armenians.
However, he reiterated Turkey's stance that the killings were not genocide.
"To reduce everything to a single word, to put responsibility through generalisations on the Turkish nation alone... is legally and morally problematic," he said.
Mr Davutoglu did acknowledge the deportations, saying: "We once again respectfully remember and share the pain of grandchildren and children of Ottoman Armenians who lost their lives during deportation in 1915."
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.