The presidents of Russia and France have joined other leaders at ceremonies commemorating the massacre 100 years ago of 1.5 million Armenians by Ottoman Turks, an event which remains a diplomatic sore point for both sides.
The annual April 24 commemorations mark the day when 250 Armenian intellectuals were rounded up in what is regarded as the first step of the massacres.
An estimated 1.5 million died in the massacres, deportations and forced marches that began in 1915 as Ottoman officials worried that the Christian Armenians would side with Russia, its enemy in the First World War.
The event is widely viewed by historians as genocide but modern Turkey, the successor to the Ottoman Empire, vehemently rejects the charge, saying that the toll has been inflated, and that those killed were victims of civil war and unrest.
On the eve of the centennial, Turkish president Recep Tayyip Erdogan insisted that his nation's ancestors never committed genocide.
Russian president Vladimir Putin, French president Francois Hollande and other dignitaries assembled at the Tsitsernakaberd memorial complex in the Armenian capital, Yerevan.
Each leader walked along the memorial with a single yellow rose and put it into the centre of a wreath resembling a forget-me-not, a flower that was made the symbol of the commemoration.
"We will never forget the tragedy that your people went through," Mr Hollande said.
France is home to a sizeable Armenian community. Among the French Armenians at Yerevan was 90-year old singer Charles Aznavour, who was born in Paris to a family of massacre survivors.
Mr Putin used his speech to warn of the dangers of nationalism as well as "Russophobia" in a clear dig at the West-leaning government in Ukraine.
Earlier this month, Turkey recalled its ambassadors to Vienna and the Vatican after Austria and Pope Francis described the killings as genocide. The European Parliament has also provoked Turkey's ire by passing a non-binding resolution to commemorate "the centenary of the Armenian genocide".
Armenian president Serge Sarkisian expressed hope that recent steps to recognise the massacre as genocide would help "dispel the darkness of 100 years of denial".
Armenians and Turks plan to march in Istanbul's main square to remember the Armenian intellectuals who were rounded up in the city 100 years ago and to urge the government to recognise genocide. A small nationalist group planned a protest denouncing the accusations of genocide.
Mr Sarkisian welcomed the rally in Taksim Square to honour the dead, calling them "strong people who are doing an important thing for their motherland".
Turkish prime minister Ahmet Davutoglu earlier this week issued a message of condolence to the descendants of the victims, without calling the killings genocide.
Volkan Bozkir, minister in charge of Turkey's relations with the European Union, attended a service at the Armenian Patriarchate in Istanbul to honour the dead in the 1915 massacre - a first by a Turkish government official.
"We respect the pain experienced by our Armenian brothers," Mr Bozkir said. "We are in no way opposed to the commemoration of this pain. We felt indebted to attend this service."
In Berlin, Germany's parliamentary speaker said the slaughter was genocide, adding that Germany's own past made it important to speak out.
Norbert Lammert told parliament: "We Germans cannot lecture anyone about dealing with their past, but we can through our own experiences encourage others to confront their history, even when it hurts."