In the capital Yerevan, Armenian President Serzh Sarkisian and first lady Rita Sarkisian laid a wreath at a hilltop memorial at the start of a solemn ceremony commemorating the mass killings that began in 1915.
Under a leaden sky shedding rain, foreign diplomats followed, each holding a yellow rose to put into the wreath laid at the foot of a monumental 44-metre (144-foot) needle, symbolizing the nation's rebirth.
French President Francois Hollande and Russia's Vladimir Putin, who are among a handful of world leaders to visit for the anniversary, then joined the ceremony.
Hundreds of thousands of Armenians will later join a procession to the genocide memorial -- Armenia's most visited landmark -- carrying candles and flowers to lay at the eternal flame.
Members of the massive Armenian diaspora that came into existence as a result of the slaughter that went on until 1917 were also to commemorate the sombre anniversary in cities around the world.
The patchy list of foreign dignitaries attending commemorations in Yerevan highlights the lack of international consensus over Armenia's bid to get the massacres recognised internationally as a genocide.
Many foreign leaders shied away for fear of upsetting Ankara.
More than 20 nations -- including France and Russia -- have so far recognized 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 in Berlin commemorating the bloodletting.
Gauck said on Thursday that the then German empire -- the Ottoman Turkey's ally in WWI -- bore "shared responsibility, possibly shared guilt for the genocide."
Germany deployed soldiers who took part in "planning and, in part, carrying out the deportations", he said.
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 people were killed, but mostly due to war and starvation, and rejects the use of the term "genocide".
US President Barack Obama on Thursday would only go so far as to describing the World War I massacres as "terrible carnage".
- Mass canonization -
In an unusual ceremony on Thursday, the Armenian Church conferred sainthood on those massacred by Ottoman forces a century ago, in what was believed to be the biggest canonization service in history.
The ceremony outside Armenia's main cathedral at Echmiadzin, close to Yerevan, ended at 7:15 pm local time, or 19:15 according to the 24-hour clock, to symbolize the year when the massacres started.
"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," Karekin II, the head of the Armenian Apostolic Church said at the ceremony.
"The canonization of the martyrs of the genocide brings life-giving new breath, grace and blessing to our national and ecclesiastical life."
- 'Reconciliation' -
Ex-Soviet Armenia and the huge Armenian diaspora worldwide have battled for decades to have the World War I massacres at the hands of the Ottoman forces between 1915 and 1917 recognized as a targeted genocide.
But modern Turkey, which was born out of the ashes of the Ottoman Empire, has refused to do so, and relations remain frozen to this day.
Ankara says 300,000 to 500,000 Armenians and at least as many Turks died in civil -- rather than religious -- strife when they say Armenians rose up against their Ottoman rulers and sided with invading Russian troops.
In a rare interview with Turkish television broadcast Thursday, President Sarkisian expressed hope the two countries could mend fences.
"It is obvious that a reconciliation between the two peoples will have to come about through Turkey recognizing the genocide," he told CNN-Turk.
Ahead of the ceremonies, Turkey kicked up a diplomatic storm, condemning growing "racism" in Europe.
Earlier this month Ankara also recalled its envoy to the Vatican after Pope Francis described the killings as "the first genocide of the 20th century."
Turkey will on Friday host world leaders to mark the 100th anniversary of the start of the Battle of Gallipoli, a day earlier than the actual start of fighting.
Sarkisian has accused Ankara of deliberately "trying to divert world attention" from the Yerevan commemorations.--AFP