Russian President Vladimir Putin, French President Francois Hollande and other dignitaries assembled at the Tsitsernakaberd memorial complex in Yerevan on Friday morning.
They walked along the memorial with a single yellow rose and lay it in the center of a wreath resembling a forget-me-not, a flower declared as a symbol of the commemoration.
"We will never forget the tragedy that your people went through," the French president 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," Hollande added.
President Vladimir Putin said the world should never allow a repetition of the genocide of more than 1.5 million Armenians.
"There is no and cannot be any justification for the mass murder of people," Putin underlined.
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 recognize 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.
Centennial genocide commemorations in Armenia
In a procession to a hilltop memorial on the outskirts of the capital Yerevan, Armenians carried candles and flowers to lay at an eternal flame.
The Armenian Church conferred sainthood on those killed in a genocide by Ottoman forces a century ago, in what was believed to be the biggest canonization service in history.
More than 20 nations have recognized the Armenian genocide so far, including Russia and France.
German President Joachim Gauck also used the term genocide for the first time in his speech in Berlin on Thursday.
US President Barack Obama described the World War I massacre of Armenians as "terrible carnage." However, he didn't use the term genocide.
"The Armenian people of the Ottoman Empire were deported, massacred and marched to their deaths. Amid horrific violence that saw suffering on all sides, one and a half million Armenians perished," Obama said on Thursday in a carefully worded statement.
Most historians believe that up to 1.5 million Armenians were systematically killed by the Ottoman Empire in 1915 and 1916. But modern Turkey, the Ottoman successor state, refuses to recognize the mass killings as genocide. Ankara believes that up to 500,000 Armenian were killed, but mostly due to war and starvation.
Ahead of the ceremonies, on Wednesday, Turkey recalled its ambassador to Vienna in response to Austrian lawmaker's decision to use the controversial term.
A memorial service will also takeplace in Turkey on Friday. Turkey's Prime Minister Ahmet Davutoglu has said the country will "share the pain" of Armenians, but he repeated Ankara's stance that he killings were not genocide.
Commemorations expected to draw millions in Yerevan, Paris, Los Angeles and other cities worldwide.
jil/kms (AP, dpa, Reuters, AFP)