The presidents of France and Russia joined other leaders at the memorial for the victims on the outskirts of the country's capital, Yerevan.
"We will never forget the tragedy that your people went through," the French president Francois Hollande said at a Yerevan ceremony.
"Important words have already been said in Turkey, but others are still expected so that shared grief can become shared destiny," he added.
President Vladimir Putin urged the world should never allow a repetition of the genocide of more than 1.5 million Armenians, a figure which Turkey disputes.
"There is no and cannot be any justification for the mass murder of people," Putin said.
Speaking at the ceremony, Armenian President Serge Sarkisian thanked world leaders for attending a ceremony to mark 100 years since the start of the massacre of Armenians by Ottoman Turks.
Sarkisian also expressed hope that recent steps to recognise the massacre as genocide would help "dispel the darkness of 100 years of denial."
"I am grateful to all those who are here to confirm your commitment to human values, to say that nothing is forgotten," the Armenian president said to a standing ovation from the audience, which also included Cyprus President Nicos Anastasiades and Serbian President Tomislav Nikolic.
On Wednesday, Turkey recalled its ambassador to Vienna in protest at the Austrian parliament's decision to call the massacre "genocide."
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 strongly objects to the use of the term genocide to describe the killings and the issue has soured relations between the nations.
Turkey accepts that atrocities were committed but argues there was no systematic attempt to destroy the Christian Armenian people. Turkey says many innocent Muslim Turks also died in the turmoil of war.
On Thursday the Armenian Church canonised the 1.5 million people it says were killed in the massacres and deportations.
Dr Harry Hagopian, consultant for the Armenian Church in the Middle East speaking to Premier's Hannah Tooley:
