If bullying is ever going to stop, we must leave aggressions behind, he said, adding that "bullying is an aggression that hides a profound cruelty."
"The world is cruel. Wars are monuments of cruelty," he said, and pulled out from his pocket graphic pictures sent to him by a nun living in an African country torn apart by civil war.
Showing the pictures to the participants, Francis lamented how they depicted grotesque images such as a child with their throat cut and another "butchered in the head."
"If this happens, how will bullying not? It's the same cruelty," he said, explaining that if we want to build a better world, we must first eliminate "all forms of cruelty. War is a form of cruelty."
Instead of aggression, we must gain the ability to listen to one another and to dialogue, rather than argue, he said, telling attendees "don't be afraid of dialogue," because with dialogue "everyone wins, no one loses."
Pope Francis also stressed the importance of leaving one's pride and superiority behind, because these attitudes "always end badly."
The world today "needs to lower the level of aggression, it needs tenderness, it needs to listen, it needs to walk together," he said, and, pointing to the photos, said added that "this is happening today because all these attitudes are lacking."
Francis was then presented with several other initiatives before receiving a signed copy of all the commitments made by participants during the three-day gathering.
Mention of the second interreligious Match for Peace, to be played June 10 in Rome's Olympic Stadium and which draws together major soccer stars from around the world, was also mentioned.
The Pope closed the gathering by offering his gratitude to participants for their prayers and work, and praying for youth around the world.
Elise Harris was senior Rome correspondent for CNA from 2012 to 2018.