The archbishop said he had prayed in Warsaw with the youth attending WYD at the tomb of Fr. Popiulusko, a priest who was assassinated in 1984 during the communist regime.
"The Catholic Church cannot take weapons other than those of prayer and brotherhood among men," the Rouen archbishop said, explaining that he would be returning to his diocese where the people are "very much in shock."
"I leave here hundreds of young people who are the future of humanity, the true ones," he said. "I ask them not to give in to the violence," but instead "become apostles of the civilization of love."
French prime minister Manuel Valls decried the "horror" of the "barbaric attack," writing on Twitter: "The whole of France and all Catholics are wounded. We will stand together."
Tuesday's killing comes little over a week after a teenage Afghan Islamist went on an axe rampage in Würzburg, Germany, which left several passengers severely wounded. More recently, just last Saturday, around 80 people were killed and 230 people wounded after two explosions struck the Afghan city of Kabul.
The Vatican's July 26 statement came in response to the "terrible new news" of the deadly hostage situation in a church in Rouen, the latest in "a series of violence which, in recent days has shocked us," and caused "immense suffering and worry."
In less than two years, France has witnessed several deadly attacks attributed to Islamic state militants, with the most recent -- and second deadliest -- taking place earlier this month. On July 14, 84 people were killed in Nice, France when a Tunisian man intentionally drove a large truck through a crowded beach street at high speed during a Bastille Day celebration.
On Nov. 13, 2015, nearly 130 people were killed in a series of attacks throughout Paris. In January of that same year, a total of 12 people were killed in the French capital after terrorists stormed the offices of the Charlie Hebdo satirical magazine.
During an address at WYD for the launch of DoCat, a new Catholic social doctrine app for young people, the Archbishop of Manila, Cardinal Luis Antonio Tagle, responded to the attacks.
"We want to express also our unity, our communion of prayer, even of sorrow, with the people of France," he said.
Although little is yet known about the incident, he said we are nonetheless "shocked, we are saddened, and we pray for the people of France."
(Story continues below)
Subscribe to our daily newsletter