"I thank you in the name of all Christians," Rouen Archbishop Dominique Lebrun told them. "In this way you are affirming that you reject death and violence in the name of God."
"Being united is a response to the act of horror and barbarism."Notre Dame church in southwestern Bordeaux also welcomed a Muslim delegation, led by the city's top imam, Tareq Oubrou.
"It's an occasion to show (Muslims) that we do not confuse Islam with Islamism, Muslim with jihadist," said Reverend Jean Rouet.Muslims were responding to a call by the French Muslim council CFCM to show "solidarity and compassion" over the priest's murder on Tuesday.
A woman wearing a beige headscarf who sat in a back pew at a church in central Paris said:
"I'm a practising Muslim and I came to share my sorrow and tell you that we are brothers and sisters." Giving her name only as Sadia, she added softly: "What happened is beyond comprehension."At the Saint Leger church in the northern city of Lens, around 30 Muslims attended mass wearing T-shirts emblazoned with messages such as, "Terrorism has no religion or identity".
Father Hubert Renard told the congregation: "We are not alone; our Muslim brothers are here too."
Many were moved to tears during 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.
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.
Also on Sunday, dozens of prominent Muslims published a joint letter pledging: "We, French and Muslim, are ready to assume our responsibilities."