Muslims in France and Italy attended Catholic masses on Sunday in a gesture of solidarity after the killing of a French priest in Normandy by Islamist militants.

The knife-wielding attackers burst into a Catholic church service in Saint-Etienne-du-Rouvray, western France, on July 26, forced a 85-year-old Roman Catholic priest to his knees and slit his throat. The attack was claimed by Islamic State.

Among those who attended Catholic services was the rector of the Great Mosque of Paris, Dalil Boubakeur, who is also the president of the French Council of the Muslim Faith. He attended a morning service in Notre-Dame cathedral in central Paris on Sunday.

The Basilica of Saint-Denis, outside Paris, also gathered hundreds of Catholics but also a large number of Muslims and people of other confessions who showed up after religious authorities in France called on the population to express sympathy to the Catholic community.

"I'm very pleased that we invited Muslims. We also share their pain, the pain of all those who suffer, in every way," Danielle Ludon, a Catholic woman who attended mass, told Reuters.

"The sentiments expressed were very, very strong. Some of them were very poignant," she said.

Among those who attended the service was a Muslim woman called Hayat, who came with her children and husband.

"This was basically a message of unity, aside from peace, it was really about unity," she said.

At the Gothic cathedral in Rouen, only a few miles from where Fr Hamel was killed last Tuesday, churchgoers were cheered by their Muslim visitors.

Interviewed outside the cathedral in Rouen, one worshiper, Jacqueline Prevot, told AP it was "a magnificent gesture."

"Look at this whole Muslim community that attended Mass," she said. "I find this very heartwarming."

French television broadcast scenes of interfaith solidarity, with Muslims and Jews in the front rows of Catholic cathedrals in Lille, Calais and the Basilica of St Denis.

In Italy the head of Italy's Union of Islamic communities, Izzedin Elzir, called on his colleagues to "take this historic moment to transform tragedy into a moment of dialogue".

Imams representing their Muslim communities also took part in mass in many Italian cities and towns including Rome's Santa Maria in Trastevere and Milan's Santa Maria in Caravaggio.

Ahmed El Balazi, the imam of the Vobarno mosque in Italy's Lombard province of Brescia, told AP terrorists were "tainting our religion and it is terrible to know that many people consider all Muslim terrorists. That is not the case."

He continued: "Religion is one thing. Another is the behaviour of Muslims who don't represent us."

Italian Foreign Minister Paolo Gentiloni said on Twitter: "Thank you to all those Italians of Islamic religion who direct their communities along the path of courage against fundamentalism."

Additional reporting by Reuters.