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ROUEN, France — In a gesture of solidarity following the gruesome killing of a French priest, Muslims on Sunday attended Catholic Mass in churches and cathedrals across France and Italy.

An Associated Press reporter at the scene said that a few dozen Muslims gathered at the towering Gothic cathedral in Rouen, near Saint-Etienne-du-Rouvray where the 85-year-old Rev. Jacques Hamel had his throat slit by two teenage Muslim fanatics on Tuesday.

"We are very moved by the presence of our Muslim friends, and I believe it is a courageous act that they did by coming to us," Dominique Lebrun, the archbishop of Rouen, said after the service.

A Catholic monk talks with a Muslim worshiper in front of Saint-Pierre-de-lAriane Church before a mass Sunday in Nice, France. Jean Christophe Magnenet / AFP - Getty Images

Some of the Muslims sat in the front row, across from the altar. Among the parishioners was one of the nuns who was briefly taken hostage at Hamel's church after the priest was killed. She joined her fellow Catholics in turning to shake hands or embrace the Muslim churchgoers after the service.

Outside the church, a group of Muslims were applauded when they unfurled a banner: "Love for all. Hate for none."

Churchgoer Jacqueline Prevot said that the attendance of Muslims was "a magnificent gesture."

Related: Priest Killed in ISIS-Linked Hostage Drama at French Church

"Look at this whole Muslim community that attended Mass," she said. "I find this very heartwarming; I am confident. I say to myself that this assassination won't be lost, that it will maybe relaunch us better than politics can do; maybe we will react in a better way."

Many of the Muslims who attended the service in Rouen — including those with the banner — were Ahmadiyya Muslims, a minority sect which differs from mainstream Islam in that it doesn't regard Muhammad as the final prophet.

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Similar interfaith gatherings were repeated elsewhere in France, as well as in neighboring Italy.

At Paris' iconic Notre Dame cathedral, Dalil Boubakeur, the rector of the Mosque of Paris, said repeatedly that Muslims want to live in peace.

A Catholic monk welcomes Muslim worshipers in Saint-Pierre-de-lAriane Church before a mass Sunday in Nice, France. Jean Christophe Magnenet / AFP - Getty Images

"The situation is serious," Boubakeur told BFMTV. "Time has come to come together so as not to be divided."

In Italy, the secretary general of the country's Islamic Confederation, Abdullah Cozzolino, spoke from the altar in the Treasure of St. Gennaro chapel next to Naples' Duomo cathedral. Three imams also attended Mass at the St. Maria Church in Rome's Trastevere neighborhood, donning their traditional dress as they entered the sanctuary and sat down in the front row.

Related: French Church Attack: How Should Known Jihadis Be Tracked?

Mohammed ben Mohammed, a member of the Union of Islamic Communities in Italy, said that he called on faithful in his sermon Friday "to report anyone who may be intent to damage society. I am sure that there are those among the faithful who are ready to speak up."

Ahmed El Balzai, the imam of the Vobarno mosque in the Lombard province of Brescia, said he did not fear repercussions for speaking out.

"I am not afraid. ... These people are tainting our religion and it is terrible to know that many people consider all Muslim terrorists. That is not the case," El Balazi said. "Religion is one thing. Another is the behavior of Muslims who don't represent us."

Foreign Minister Paolo Gentiloni thanked Italian Muslims for their participation, saying they "are showing their communities the way of courage against fundamentalism."

As in France, Italy is increasing its supervision of mosques. Interior Minister Angelino Alfano told the Senate this week that authorities were scrutinizing mosque financing and working with the Islamic community to ensure that imams study in Italy, preach in Italian and are aware of Italy's legal structure.

Meanwhile the Paris prosecutor's office said it has requested that a cousin of one of the two 19-year-olds who slit the priest's throat should be charged with participating in "a terrorist association with the aim of harming others."

In a statement it said it appeared 30-year-old Frenchman Farid K. "knew very well, if not of the exact place or time, of his cousin's impending plans for violence."

The office added that a Syrian refugee detained in the wake of the attack was released Saturday.