The French Catholic Bishops have called for a day of fasting on Friday in response to the murder yesterday of the 86-year-old priest Jacques Hamel in Normandy.

"I invite all the Catholics of France to participate in a day of fasting and prayer for our country and for peace in the world this Friday," said Msgr Olivier Ribadeau Dumas, secretary general of the French Bishops Conference.

Speaking to journalists in Krakow at the World Youth Day gathering, Dumas said: "What happened in France had happened in other countries before, and actually we see Christians laying down their lives in the interests of their faith. They die because they are objects of hate and this for a fact gives us an additional motivation to live the life of fraternity we are called to."

Speaking after the attack on the church in Saint-Etienne-du-Rouvray, where the priest was celebrating Mass with two nuns and two other worshippers, Dumas said: "A priest is a symbol of peace and fraternity, and he was an old priest, more than 50 years as a priest in France, so...we are sad and we are shocked by this".

He went on to call for a maintained dialogue with people of different faiths in France: "[We] are also something which is very strong. We want to maintain and develop dialogue between the different people in our country. We need peace, we need fraternity, we need to build a society where people love each other, and we will continue this path. The Catholic Church in France wants that...We should see the horizon, the horizon of peace, of joy, brotherhood and prayer. We are rooted in our faith and in Christ and we believe that evil and violence will not have the upper hand."

Around 300 youth gathered in Poland are from the Diocese of Rouen - site of the church where Hamel was attacked - out of 3,000 French pilgrims in total.

Despite its start being overshadowed by the killing, Dumas stressed that World Youth Day needs to go forward "with even greater intensity."

He noted that many Muslims are also killed around the world. "Many more Muslims than Christians are killed because they are Muslims, so we pray and we are going to pray at Mass in a few minutes for peace and for all those who are killed because they believe in God," he said.

On fostering dialogue between people of different faiths, he added: "It's a bit hard, difficult, so we do it with hope. It's a Christian attitude because we think that the Catholic religion can involve all of our society, and that is very important for us. Violence is not the answer, the only answer is really love. We cannot do anything else. Love, love and love. Dialogue and dialogue. And also have mercy for all those who are totally distracted by violence."

Pope Francis has responded to the murder by condemning "every form of hatred" and the Vatican spokesman said the Pope "participates in the pain and horror of this absurd violence."

Archbishop Dominique Lebrun of Rouen also reacted to the attack. Also in Krakow, he said: "I cry out to God, with all men of good will. And I invite all non-believers to unite with this cry ... The Catholic Church has no other weapons besides prayer and fraternity between men."