Reporters on the scene said that between 100 and 200 Muslims gathered at the towering Gothic cathedral in Rouen, only a few miles from Saint-Etienne-du-Rouvray, where the 85-year-old Rev. Jacques Hamel was killed by two teenage attackers on Tuesday.
"We're very touched," Archbishop Dominique Lebrun told broadcaster BFMTV. "It's an important gesture of fraternity . They've told us, and I think they're sincere, that it's not Islam which killed Jacques Hamel."
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Outside the church, a group of Muslims were applauded when they unfurled a banner: "Love for all. Hate for none."
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.
"The situation is serious," he 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. He said there was a "need of dialogue, more affirmation of shared values of peace, of solidarity, of love, out of respect for our one God, merciful and compassionate."
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.
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."
"Mosques are not a place in which fanatics become radicalized. Mosques do the opposite of terrorism: They diffuse peace and dialogue," he added.
Foreign Minister Paolo Gentiloni thanked Italian Muslims for their participation, saying they "are showing their communities the way of courage against fundamentalism."
Like 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.