The Presidents of France, Francois Hollande and Russia,Vladimir Putin, joined other leaders at the memorial for the victims on the outskirts of the capital, Yerevan.
Armenia said up to 1.5 million people died, a figure disputed by Turkey.
Armenian President, Serzh Sarkisian and First Lady, Rita Sarkisian, laid a wreath at a hilltop memorial at the start of a solemn ceremony, commemorating the mass killings that began in 1915 during World War 1.
President Sarkisian expressed hope that recent steps to recognise the massacre as genocide would help “dispel the darkness of 100 years of denial”.
Each foreign diplomat held a yellow rose to put into the wreath laid at the foot of a monumental 44-metre needle, symbolising the nation’s rebirth.
Trying To Divert World Attention
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 said many innocent Muslim Turks also died in the turmoil of war.
A memorial service will also be held in Turkey on Friday and its prime minister, Ahmet Davutoglu, has said the country will “share the pain” of Armenians. But he reiterated Turkey’s stance that the killings were not genocide.
Turkey is also hosting ceremonies on Friday to mark the 100th anniversary of the start of the Battle of Gallipoli.
However, the actual fighting there began on 25 April, and President Sarkisian has accused Turkey of “trying to divert world attention” from the Yerevan commemorations.