The leaders of France and Russia joined ceremonies marking the centenary of the massacre of some 1.5 million Armenians by Ottoman forces, a hugely emotional event that remains a diplomatic minefield.
During a commemoration at a hilltop memorial in the Armenian capital Yerevan, French President Francois Hollande urged modern day Turkey to recognise the massacre as genocide, saying he bowed in memory of the victims.
"Important words have already been said in Turkey, but others are still expected, so that shared grief can become shared destiny," Hollande told an audience that also included the leaders of Cyprus and Serbia and delegates from some 60 countries.
President Vladimir Putin said Russia stood shoulder to shoulder with ex-Soviet Armenia, one of Moscow's closest allies.
"There... cannot be justification for mass murder of people," said Putin, who also described the killings as "genocide" in remarks that drew ire from Ankara.
Muslim Turkey, which was born out of the ashes of the Ottoman Empire, has refused to call the slaughter of Christian Armenians genocide.
Ankara concedes that up to 500,000 people were killed, but says this was mostly due to fighting and starvation during World War I, when Armenians rose up against their Ottoman rulers and sided with invading Russian troops.
Also on Friday, Turkey hosted leaders from the former Allied powers of World War I to pay tribute to the tens of thousands killed in the Battle of Gallipoli. Armenians have accused Ankara of timing the commemorations to deliberately overshadow the Yerevan ceremonies.
In Yerevan, leaders walked through the rain to lay flowers at a memorial commemorating the victims. Each put a yellow rose at the centre of a wreath resembling a forget-me-not, a flower that has become a symbol of the massacres.
Hundreds of thousands joined a procession to the genocide memorial -- the country's most visited landmark -- carrying candles and flowers to lay at the eternal flame.
"I am grateful to all those who are here to once again confirm your commitment to human values, to say that nothing is forgotten, that after 100 years we remember," Armenian President Serzh Sarkisian told his guests.
- 'Watershed moment' -
But the patchy list of foreign dignitaries attending the commemorations highlighted a lack of international consensus over Armenia's bid to get the massacres recognised internationally as a genocide.
Germany became the latest country to use the term, with President Joachim Gauck saying Thursday that his country, then an ally of the Ottomans, bore partial blame for the bloodletting.
More than 20 nations have so far recognised the killings as genocide, a definition supported by numerous historians but vehemently opposed by Turkey.
In a first for a Turkish government official, European affairs minister Volkan Bozkir attended a mass at an Armenian church in Istanbul to mark the 1915 massacres.
President Recep Tayyip Erdogan also repeated his "condolences" to the victims' descendants as around 100 people staged a rally in Istanbul, demanding the government recognise the killings as genocide.
"Our hearts remain wide open to the grandchildren of the Ottoman Armenians all around the world," Erdogan said in a message.
But in Yerevan, anger still simmered among Armenians as tens of thousands held an annual torch-lit march through the centre of the city.
Demonstrators sang patriotic songs and burned a huge Turkish flag during the traditional procession staged every year on April 24 and led by the youth wing of the nationalist Dashnaktsutyun party.
"I hope that the centenary will be a watershed moment in the Armenians' struggle for the recognition of the genocide," said Ani Sahakyan, 37, a Yerevan resident.
"We demand that Turkey recognise its guilt and make an apology," said Sevan Gedelekian, an ethnic Armenian from Lebanon.
- Commemorations and controversy-
The Armenian Church on Thursday conferred sainthood on the victims of the massacres in what was believed to be the biggest canonisation service in history.
From New York to Paris to Beirut, members of the massive Armenian diaspora that came into existence as a result of the slaughter that went on until 1917 also commemorated the anniversary.
Tens of thousands of protestors marched along Sunset Boulevard through Hollywood to the Turkish consulate.
About 500 people gathered for a Mass in the Armenian Cathedral of Saint James in Jerusalem, while another 200 people held a rally near the Turkish consulate in East Jerusalem.
In France, which has a huge Armenian diaspora of 600,000 people, thousands tuned out to remember the killings.
"We want Turkey to fall in step with its history, for it to recognise the genocide," said Jacques Donabedian, co-president of the Coordination Council of Armenians in France.
In Tehran, over 1,000 people protested outside the Turkish embassy, holding placards that read "Recognise Armenian genocide" and "Turkey don't deny".
Ahead of the anniversary, Turkey kicked up a diplomatic storm, condemning growing "racism" in Europe and recalling its ambassador to Vienna in response to Austrian lawmakers' decision to use the word genocide.
US President Barack Obama on Thursday went only so far as to describe the massacres as "terrible carnage".