More than 100 Muslims were among the 2,000 faithful who packed the 11th-century Gothic cathedral of Rouen, near the Normandy town where two teenagers slit the throat of 85-year-old Father Jacques Hamel.
"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."
A few policemen and soldiers stood guard outside but did not conduct searches, seeking to reassure a jittery population after the second attack in less than a fortnight.
In the southern city of Nice, where a jihadist carried out a rampage in a truck on July 14, claiming 84 lives, local imam Otaman Aissaoui led a delegation of Muslims to a Catholic mass.
"Being united is a response to the act of horror and barbarism," Aissaoui said.
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.
Said a woman wearing a beige headscarf who sat in a back pew at a church in central Paris: "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. Other joint services were held in Milan, Naples and Palermo, Sicily.
The killing of Father Hamel fanned fears of religious tensions in France and renewed recriminations over perceived security lapses.
Both of the 19-year-old attackers -- Adel Kermiche and Abdel Malik Petitjean -- had been on the intelligence services' radar and had tried to go to Syria.
Killer's cousin charged
Prime Minister Manuel Valls called Sunday for a new "pact" with the Muslim community in France, Europe's largest with around five million members.
Also Sunday, dozens of prominent Muslims published a joint letter pledging: "We, French and Muslim, are ready to assume our responsibilities."
Meanwhile, Petitjean's 30-year-old cousin was charged with "criminal association in connection with terrorism", the Paris public prosecutor said.
The suspect, named as Farid K., "was fully aware of his cousin's imminent violent action, even if he did not know the precise place or day," the prosecutor said in an earlier statement.
Media reports say Petitjean and Kermiche met through the encrypted messaging app Telegram.
In a separate case Sunday, 20-year-old Jean-Philippe J. was charged with trying to travel to Syria with Petitjean last month.