He is among world leaders who have paid tribute after Catholic priest Fr Jacques Hamel, 84, was murdered and a member of the congregation seriously injured in Saint-Etienne-du-Rouvray, south of Rouen, Normandy.
The Vatican said: “We are particularly struck because this horrible violence has occurred in a church — a sacred place where we pronounce God’s love — with the barbaric murder of a priest and worshippers affected.”
French president Francois Hollande travelled to the site of the attack as forensic officers began their investigations.
In a tweet addressed “to the families of the victims and all Catholics in France”, Mr Hollande offered “solidarity and compassion”.
Confirming Fr Hamel’s death, the Archbishop of Rouen, Dominique Lebrun, who cut short a visit to Poland to return to his diocese, said: “I cry to God with all men of good will. The only arms which the Catholic Church can take up are prayer and brotherhood between men.”
French prime minister Manuel Valls said: “All of France and all Catholics are bruised. We stand together.”
Chief rabbi Mirvis offered “prayers with our Catholic friends” on Twitter.
He said: “Attack in Normandy a despicable desecration of the sanctity of human life in a place of peaceful worship.”
In England, the Archbishop of Canterbury Justin Welby said: “Evil attacks the weakest, denies truth and love, is defeated through Jesus Christ. Pray for France, for victims, for their communities.”
British prime minister Theresa May said Britain would “stand shoulder to shoulder with France”, adding that a terrorist attack in the UK was “highly likely”.
She said: “What is necessary is for us all to work together and stand shoulder to shoulder with France. We offer them every support we have in dealing with this issue and this threat that they, and the rest of us, are facing.
“But on one thing, I think, we are all absolutely clear, and that is the terrorists will not prevail.
“They are trying to destroy our way of life. They are trying to destroy our values. We have shared values and those values will win through and the terrorists will not win.”