YEREVAN, Armenia — Thousands of Armenians laid flowers around an eternal flame here on Friday in memory of the 1.5 million people killed in Ottoman Turkey a century ago. The ceremony was one of many around the world marking the 100th anniversary of what historians call a genocide.

The legacy of those mass killings and forced deportations, which Turkey still insists were not genocide, remains a source of bitter enmity and continues to roil politics in Asia Minor and beyond.

On an ashen gray day here in the Armenian capital, President Serzh Sargsyan was joined by international delegations that included Presidents Vladimir V. Putin of Russia and François Hollande of France at the Tsitsernakaberd Memorial Complex, the main monument to the Armenian genocide, on a hilltop overlooking the city.

Mr. Sargsyan, in his opening remarks, described the killing of Armenians as “unprecedented in terms of volume and ramifications” at that point in history.