Ahead of the vigil, Greg Burke, Senior Communications Advisor for the Holy See's Secretary of State, said Pope Francis wanted "to keep this issue alive to make sure everything possible could be done to put an end to the violence, and not to increase it."
Burke said the response to the vigil has been "really positive" from Catholics, other Christians and non-Christians.
Jeff Runter certainly found it to be so. He said he hoped that the "sense of peace and the oneness of humanity" he felt in the square can "resonate with all of the leaders of the world."
"What's going on in Syria is terrible," he added, "and there are no easy solutions. But perhaps this will help."
Italian Gabrielle Palmieri, 35, felt similarly.
"Today is a very important day," he said. "Pope Francis' voice is very powerful. Not only Christians but Muslims and Jewish people and non-believers listen to his word. Maybe new ways of peace are coming."
As the organ played in anticipation of the vigil's beginning, Sister Michaela of the Handmaidens of the Child Jesus sat quietly in her chair, reading the prayer book given out for the service.
"I'm here for peace," she said, "for prayer, for peace."
Via di Conciliazione, the street leading to the square, was packed with people joining the vigil as twilight came. Families with small children joined with priests, nuns, and lay faithful from around the world as Pope Francis began the joyful mysteries of the Rosary.
The crowds cheered as Pope Francis spoke of the importance of prayer and dialogue, saying, "violence and war are never the way of peace!"
Then the hundred thousand people who filled the square fell silent as Pope Francis removed his white skull cap to kneel before the Eucharist.
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The vigil continued until midnight as the participants alternated between prayers in silent adoration before the Eucharist and meditation on the scriptures read aloud.
An infant slept restfully in his mother's arms, a sign of the peace prayed for by Pope Francis and pilgrims like Angeline Martin, who hopes that the prayers "will help ensure the safety and livelihood of children throughout the world."