Pope Francis has expressed "sorrow and horror" over the suspected terror attack in a French church, the Vatican said.
He is among world leaders who have paid tribute after Catholic priest Father Jacques Hamel (84) was murdered and a member of the congregation seriously injured.
The Vatican said in a statement: "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."
Confirming Father 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 in a message on Twitter: "All of France and all Catholics are bruised. We stand together."
Taoiseach Enda Kenny said the attack was "particularly brutal".
"For centuries the Church has always been a place of sanctuary and it's particularly brutal that terror and murder have been visited upon innocent people at a time when they have been so physically vulnerable and so spiritually hopeful," he said at a press conference in Downing Street.
Prime Minister Theresa May said Britain would "stand shoulder to shoulder with France" as she spoke in Downing Street, adding a terrorist attack in the UK was "highly likely".
"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," she said.
"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."