Armenia marks the centenary of the start of mass killings of Armenians by Ottoman Turks.
Presidents Vladimir Putin and Francois Hollande joined other leaders at the memorial for the victims on the outskirts of Armenia’s capital, Yerevan.
Armenia says up to 1.5 million people died, a figure disputed by Turkey.
Turkey objects to the use of the term “genocide” to describe the killings.
The issue has soured relations between Armenia and Turkey.
Turkey accepts that atrocities were committed but argues there was no systematic attempt to destroy the Christian Armenian people. It says many innocent Muslim Turks also died in the turmoil of war.
A memorial service will also be held in Turkey on April 24 and PM Ahmet Davutoglu has said the country will “share the pain” of Armenians.
However, Ahmet Davutoglu reiterated Turkey’s stance that the killings were not genocide.
However, the actual fighting in Gallipoli began on April 25, and Armenian President Serzh Sargsyan has accused Turkey of “trying to divert world attention” from the Yerevan commemorations.
After the flower-laying ceremony in Yerevan, Serzh Sargsyan addressed the guests, saying: “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.”
France’s President Francois Hollande said: “We will never forget the tragedies that your people have endured.”
April 24 marks the 100th anniversary of the day the Ottoman Turkey authorities arrested several hundred Armenian intellectuals in Constantinople, today’s Istanbul, most of whom were later killed.
The Armenians regard this as the beginning of the Ottoman policy of mass extermination of Christian Armenians suspected of supporting Russia, the Ottoman Empire’s World War One enemy.
Russian President Vladimir Putin is also among those attending the ceremonies.
President Barack Obama issued a carefully worded statement for the anniversary, referring to “one of the worst atrocities of the 20th Century”, without using the term “genocide”.
During his 2008 presidential election campaign, then senator Barack Obama had vowed to “recognize the Armenian genocide” and in his new statement said: “I have consistently stated my own view of what occurred in 1915, and my view has not changed.”
However, Barack Obama’s phrasing has angered Armenian Americans.
Bryan Ardouny, executive director of the Armenian Assembly of America, said in a statement: “President Obama’s exercise in linguistic gymnastics on the Armenian genocide is unbecoming of the standard he himself set and that of a world leader today.”
On April 23, the Armenian Church canonized the 1.5 million people it says were killed in the massacres and deportations.
The church said it wanted to proclaim the martyrdom of those who died for their faith and homeland.
After the ceremony, bells tolled in Armenian churches around the world.
Also on April 23, German President Joachim Gauck described the killings as genocide, on the eve of a debate in the German parliament on the issue.
Earlier this month, Turkey recalled its envoy to the Vatican after Pope Francis also used the word “genocide” in a reference at a Mass at St Peter’s Basilica.
France has been a strong advocate of recognizing the killings as genocide and President Francois Hollande has pushed for a law to punish genocide denial. The issue has strained Franco-Turkish relations.