YEREVAN, Armenia — Hundreds of thousands of Armenians laid flowers Saturday at a monument to the victims of mass killings by Ottoman Turks, marking the 95th anniversary of the start of the slaughter. US President Barack Obama called it "one of the worst atrocities of the 20th century."Historians estimate that up to 1.5 million Armenians were killed by Ottoman Turks around the time of World War I. Turkey denies that the deaths constituted genocide, saying the toll has been inflated and those killed were victims of civil war and unrest.Residents of Yerevan and other regions and representatives of the Armenian diaspora marched to a monument on a hill overlooking the capital. Some carried placards such as "Nobody and nothing will be forgotten!" and "Genocide never gets old!"Armenian President Serge Sarkisian described the slaughter as "unprecedented in its scope, monstrosity and graveness of its consequences" in an address to the nation.In Asheville, North Carolina, where Obama was spending the weekend, he marked Armenian Remembrance Day by issuing a statement saying, "The indomitable spirit of the Armenian people is a lasting triumph over those who set out to destroy them."In Paris, about 1,000 people — led by famous French crooner Charles Aznavour, who is Armenia's permanent delegate to UNESCO — took part in a commemoration which climaxed at the Arc de Triomph, at the top of Champs-Elysees Avenue.The slaying began on April 24, 1915 with the rounding up of about 800 Armenian intellectuals, who were murdered. The Ottoman authorities then evicted Armenians from their homes in actions that spiraled into the mass slaughter of the Armenian population. Scholars widely view the event as the first genocide of the 20th century."We are grateful to all those in many countries, including Turkey, who understand the importance of averting crimes against humanity," Sarkisian said.Turkey has warned the US administration of diplomatic consequences if it fails to prevent the passage of a congressional resolution that would brand the killings of Armenians genocide. The Foreign Affairs Committee of the US House of Representative's last month passed a resolution declaring the killings genocide, but it is unclear if the full House will vote on it.Countries recognizing the killings as genocide include Uruguay, Chile, Argentina, Russia, Canada, Lebanon, Belgium, Greece, Italy, the Vatican, France, Switzerland, Slovakia, the Netherlands, Poland, Lithuania and Cyprus.Last month, Sweden's parliament narrowly approved a resolution recognizing the slaying of Armenians as genocide.Obama's statement, which did not use the word "genocide," said: "I have consistently stated my own view of what occurred in 1915, and my view of that history has not changed. It is in all of our interest to see the achievement a full, frank and just acknowledgment of the facts."Obama's comments angered Turkey's Foreign Ministry, which issued a statement saying: "We deeply regret this statement, which reflects an incorrect and one-sided political perception. The toughest enemy of the historical facts are subjective memory records. No nation has the right to impose its memory records on another nation. Third counties neither have a right nor authority to judge the history of Turkish-Armenian relations with political motives."In addition to tensions over the mass killings, efforts to normalize ties between Armenia and Turkey also have been thrown back by the conflict in Nagorno-Karabakh.The two countries signed agreements for reconciliation in October that would reopen their border, but neither country has ratified it. Armenia said this week it was freezing its ratification of the deal, accusing Turkey of dragging its feet by demanding the Nagorno-Karabakh dispute be settled first.Turkey closed its border with Armenia in 1993 to protest the Armenia-backed war by separatists in the Nagorno-Karabakh region; the region is an enclave within Azerbaijan but under the control of Armenian and ethnic Armenian forces.Obama praised "the Turks who saved Armenians in 1915" and said he is encouraged by "the dialogue among Turks and Armenians, and within Turkey itself, regarding this painful history. Together, the Turkish and Armenian people will be stronger as they acknowledge their common history and recognize their common humanity."