The French priest believes that World Youth Day needs to go forward "with even greater intensity."
He stressed the importance of fraternity, hope in the future, and "the desire to create a world in which violence and hate don't have any place." He said the young pilgrims "want to live it here, and they say that to all French people."
Msgr. Dumas voiced confidence in Polish authorities' efforts to ensure security at World Youth Day.
He also noted that in other parts of the world, many people are killed because they are Christians or because they are Muslims.
"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."
Msgr. Dumas said the Church most foster dialogue among different people.
"It's a bit hard, difficult, so we do it with hope," he said. "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, in his response to the attack, has condemned "every form of hatred." A Vatican statement said the Pope "participates in the pain and horror of this absurd violence."
The Vatican statement said it was especially moving that the "barbaric murder" took place in a church
Archbishop Dominique Lebrun of Rouen also reacted to the attack from Krakow.
"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," he said.
(Story continues below)
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Editor's note - updated July 26, 9:41 pm: An earlier version of this article stated that the day of fasting would be July 31. In fact, it will be July 29.