Waving flags and carrying flowers, people streamed through the Armenian capital and marched up to a massive hilltop granite memorial to hear speeches and prayers.
Weeping mourners filed into the circular block memorial, laying carnations on a flat surface surrounding a burning flame. A choir in black sang hymns as the crowd filed past, some carrying umbrellas against the sun.
The country will observe a minute of silence at 7pm and Yerevan residents will place candles on window sills in memory of the victims.
"International recognition and condemnation of genocide is a goal that not only Armenia must achieve," President Robert Kocharian was quoted as saying by the Russia's ITAR-Tass news agency.
"Armenia is ready to build normal relations with Turkey. However, the policy being pursued by Ankara is surprising not only in Armenia, but elsewhere in the world."
Ottoman authorities began rounding up intellectuals, diplomats and other influential Armenians in Istanbul on 24 April 1915, as violence and unrest grew, particularly in the eastern parts of the country.
Armenia says up to 1.5 million Armenians ultimately died or were killed over several years as part of a genocidal campaign to force them out of eastern Turkey.
Kocharian (L) said Armenia is
ready to build ties with Ankara
Turkey acknowledges that large numbers of Armenians died, but says the overall figure is inflated and that the deaths occurred in the civil unrest during the collapse of the Ottoman Empire.
France, Russia and many other countries have already declared the killings were genocide. The US, which has a large Armenian diaspora community, has not.
Turkey, which has no diplomatic ties with Armenia, is facing increasing pressure to fully acknowledge the event, particularly as it seeks membership in the European Union.
The issue is extremely sensitive in Turkey and Turks have faced prosecution for saying the killings were genocide.
Ankara earlier this month called for the two countries to jointly research the killings.
Armenian communities around the world also marked the anniversary, with church services and demonstrations. In Moscow, hundreds attended a memorial service at the construction site for an Armenian church.
In northeastern Syria, about 4000 people flocked to the city of Marqada, where thousands of Armenians are buried.
"We are here to remember our martyrs whom we should never
forget," Krikour Haydenian, a 33-year-old merchant, said.
An estimated 100,000 Armenians currently live in Syria.