ISTANBUL — On the day that Armenians from around the world came here to commemorate the killing and forced expulsion of their ancestors by the Ottoman Turks a century ago, Turkish officials chose to focus on a different centennial, that of their country’s most important military victory.

The dueling commemorations Friday illustrated the enmity between Armenians, who demand that Turkish authorities recognize as genocide the mass killing of their ancestors during World War I, and the Turkish government, which has stuck to its longstanding position that the killing of an estimated 1.5 million people was a tragic but unplanned outcome to a bloody war.

Standing in front of the train station in Istanbul where the deportation of Armenians began in 1915, descendants of the victims held up red carnations and placards reading, “genocide recognize, genocide apologize,” demanding that the Turkish authorities apologize for the genocide they have long denied.

“As Armenians we have come to Istanbul in record numbers to memorialize the brutal massacre of our family members, and to remind the Turkish government that 100 years later we are still seeking justice and accountability,” said Sarah Leah Whitson, the executive director of Human Rights Watch’s Middle East and North Africa division, who was involved in organizing the commemoration in Istanbul.