The presidents of France and Russia joined other leaders for the memorial in the Armenian capital, Yerevan.
Armenia says up to 1.5 million people died, a figure disputed by Turkey.
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.
A memorial service was held in Turkey yesterday and its Prime Minister, Ahmet Davutoglu, said the country would “share the pain” of Armenians. But he reiterated Turkey’s stance that the killings were not genocide.
Turkey is yesterday also hosting ceremonies to mark the 100th anniversary of the start of the Battle of Gallipoli.
However, the actual fighting there began on April 25, and Armenian President Serzh Sargsyan accused Turkey of “trying to divert world attention” from the Yerevan commemorations.
After a flower-laying ceremony in Yerevan, Mr Sargsyan addressed the guests, saying: “I am grateful to all those who are here to once again to confirm your commitment to human values, to say that nothing is forgotten, that after 100 years we remember.”
In his address, French President Francois Hollande said: “We will never forget the tragedies that your people have endured.”
Russian President Vladimir Putin said: “There cannot be any justification for mass murder of people. Today we mourn together with the Armenian people.”
Friday 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.
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.
US 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 Obama had vowed to “recognise 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, his 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.”
German MPs are meanwhile debating a non-binding motion on the genocide issue, a day after President Joachim Gauck used the word to describe the killings.
This month, Turkey recalled its envoy to the Vatican after Pope Francis also used the word genocide in a reference during a mass.
France has been a strong advocate of recognising the killings as genocide, and President Hollande has pushed for a law to punish genocide denial.
In Turkey yesterday, the media largely focused on Gallipoli, but one of Turkey’s oldest newspapers, Cumhuriyet, carried a surprise headline in Armenian – “Never Again”.