More than 100 Muslims were among the 2,000 faithful who packed the Gothic cathedral of Rouen near the Normandy town where two jihadi teenagers brutally murdered 85-year-old Father Jacques Hamel.
"This morning we extend a special welcome to our Muslim friends," Rouen Archbishop Dominique Lebrun said in his homily. "I thank you in the name of all Christians. In this way you are affirming that you reject death and violence in the name of God."
Outside the cathedral a few policemen and soldiers stood guard but did not conduct searches, seeking to reassure a jittery population after the second jihadist attack in less than a fortnight.
The priest's murder sparked renewed recriminations over perceived security lapses after the Bastille Day truck massacre in southern Nice claimed 84 lives. Both of the 19-year-olds attackers -- Adel Kermiche and Abdel Malik Petitjean -- had been on intelligence services' radar and had tried to go to Syria.
And the jihadist killing of a priest at his altar on Tuesday also prompted fears of possible tensions between religions in the officially secular country. The most poignant moment of Sunday's mass in Rouen was the sign of peace, a regular part of the liturgy when the faithful turn to greet each other in the pews, either shaking hands or kissing.
Archbishop Lebrun used the moment to step into the congregation and greet Muslim leaders attending, as well as three nuns who were at the church in Saint-Etienne-du-Rouvray when Hamel had his throat slit.
Prime Minister Manuel Valls called today for a new "pact" with the Muslim community in France, Europe's largest with around five million members. "Islam has found its place in France... contrary to the repeated attacks of populists on the right and far-right," he said.
"This intolerable rejection of Islam and Muslims... must be combatted -- and it is -- with the greatest strength," he said. Also today, dozens of prominent Muslims published a joint letter warning that "the risk of fracturing among the French is growing every day."
The signatories, which included academics as well as medical professionals, artists and business leaders, wrote: "We Muslims were silent because... religion is a private affair in France, (but) we must speak now because Islam has become a public affair and the current situation is intolerable."
The letter, published in the Journal du Dimanche weekly, pledged: "We, French and Muslim, are ready to assume our responsibilities." Muslims also attended Catholic masses in Italy, notably at Rome's Santa Maria di Trastevere church in response to a call by the Sant'Egidio community known for its international mediation efforts.