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An emotionally-charged mass Wednesday -- attended by French President Emmanuel Macron -- marked one year since the brutal murder of a Catholic priest whose throat was slit by two 19-year-old ISIS terrorists.

The ceremony took place outside the Saint-Etienne-du-Rouvray church, inside of which the men killed Father Jacques Hamel, 85, as he celebrated morning Mass last year.

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People attend a ceremony marking the first anniversary of the killing of French Catholic priest Jacques Hamel. (AP)

Macron said the two jihadis "have failed" in sowing a thirst for vengeance among French Catholics, and referred to Hamel as a "martyr."

"Hamel's smile has become a reminder of resistance in the face of bigotry," Macron said.

Rouen Archbishop Dominique Lebrun, who led the service, said "though he is dead, Father Jacques Hamel is still alive...hate has not triumphed, and it will never triumph."

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France's President Emmanuel Macron, center left, and sister of French Catholic priest Jacques Hamel, Roselyne, center right, attend a ceremony marking the first anniversary of the killing of French Catholic priest Jacques Hamel. (AP)

The mayor of Saint-Etienne-du-Rouvray, Joaquim Moise, spoke of the horror that the two jihadis brought to the sleepy Normandy town.

"A year ago, the population of Saint-Etienne was thrown into the whirl of emotions," said the mayor from outside the church.

"The initial disbelief was followed by fear then mixed with incomprehension, sorrow, disgust. In a horrific act, Jacques Hamel's life was taken away."

Following the ceremony, local dignitaries unveiled a 2-meter metal memorial tribute to the slain priest with writings from the Universal Declaration of Human Rights.

"This memorial is a symbol of peace and brotherhood," Moise said.

ISIS claimed responsibility for the attack last July, in which two nuns and an elderly couple were held hostage before the assailants slashed the priest's throat and seriously wounded another man.

One of the nuns slipped away and alerted authorities, and police shot and killed the attackers as they left the church. The attack was one of several ISIS-claimed assaults targeting France over the past two years.

The church reopened in October, with a special cleansing ritual and call for tolerance across religions.

The Associated Press contributed to this report.