Tens of thousands of Armenians living around the world flocked to solemn memorials and defiant protests on Friday to mark the 100th anniversary of the Armenian genocide.

The ceremonies mark the period in 1915 when Armenian intellectuals were rounded up in Ottoman-era Turkey, presaging a wave of killing that left about 1.5 million people dead. Historians consider the massacre to be the first genocide of the 20th century. Turkey denies that the killings constituted genocide, and has reacted angrily to recent moves in Europe to recognize it as such.

But Armenians around the world have vowed to preserve the memory of those killed. On Friday, they held commemorations in cities around the world, from Moscow to Marseille. The Armenian diaspora is estimated to number 7 million to 10 million people, with significant populations in Russia, France and the U.S. 

The Russian and French presidents joined other world leaders Friday at a commemoration ceremony in the Armenian capital of Yerevan on Friday. The U.S. government this week rejected calls to recognize the killings as genocide.

In Jerusalem and Tehran, Iran, some Armenians protested outside the Turkish embassy and consulate demanding that Turkey recognize the slaughter of their ancestors. "Denial of genocide... is the opening gate for another genocide," Inon Zalcman, a member of The Combat Genocide Association, told The Associated Press in Jerusalem. 

At the former prison in Istanbul where the Armenians were held in 1915, a small group of protesters held red flowers and portraits of the intellectuals detained 100 years ago. 

"They thought no one would remember the victims or the perpetrators, that it would be buried," Serge Tomassian, an Armenian who had traveled to Istanbul from California told Reuters. "What's happening today proves that's not true."

• KIRILL KUDRYAVTSEV/ Getty Images
  People attend a flower-laying ceremony at the Tsitsernakaberd Memorial, in Yerevan, Armenia on April 24, 2015. 
• KIRILL KUDRYAVTSEV/ Getty Images
  People lay flowers at the Tsitsernakaberd Memorial, on April 24, 2015, in Yerevan, as part of the Armenian genocide centenary commemoration. 
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  People wait to lay flowers at the Tsitsernakaberd Memorial, on April 24, 2015, in Yerevan.
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  Armenian honor guard soldiers lay a wreath at the Tsitsernakaberd Memorial in Yerevan on April 24, 2015.
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  A man prays during a commemoration service for the 1915 massacre of Armenians at the Armenian Patriarchate in Istanbul on April 24, 2015. 
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  People participate in the commemoration liturgy organized by the Istanbul Armenian Patriarchate to mark the 100th anniversary of the 1915 events at Kumkapi Mother Mary Church in Istanbul on April 24, 2015.
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  The Armenian Patriarchate choir sings during a commemoration service at the Armenian Patriarchate in Istanbul on April 24, 2015. 
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  People light candles during a commemoration service at the Armenian Patriarchate in Istanbul on April 24, 2015. 
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  Members of the Armenian community hold Armenian flags during a demonstration to commemorate the genocide anniversary in front of the Turkish consulate in Jerusalem on April 24, 2015. 
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  A member of the Armenian community lays a wreath at the memorial monument in Jerusalem's Old City on April 24, 2015.
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  Iranian Armenians protest near the Turkish embassy in Tehran to mark the 100th anniversary of the Armenian genocide in Tehran, Iran, on April 24, 2015. 
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  People gather to pay their respects during a ceremony at the Memorial of the Armenian Genocide in Marseille, France, on April 24, 2015.
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  People lay flowers as they gather to pay their respects during a ceremony at the Memorial of the Armenian Genocide in Marseille on April 24, 2015.
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  People gather to pay their respects during a ceremony at the Memorial of the Armenian Genocide in Marseille on April 24, 2015.
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  Youths unfurl the Armenian flag during a ceremony at the Memorial of the Armenian Genocide in Marseille on April 24, 2015.
• AP Photo/Bilal Hussein
  Armenians in Lebanon march to mark the 100th anniversary of the mass killings during World War I, in Antelias, north of Beirut, on April 24, 2015. 
• AP Photo/Bilal Hussein
  Armenians in Lebanon hold up a giant Armenian flag during a march on April 24, 2015, to mark the 100th anniversary of the mass killings in Antelias. 
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  People wear gags in central Moscow on April 24, 2015, to mark the 100th anniversary of the Armenian genocide.
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  People holding a giant Armenian flag gather in central Moscow on April 24, 2015, to mark the 100th anniversary of the Armenian genocide.