"With utmost firmness, I condemn the use of chemical weapons. I tell you that those terrible images from recent days are burned into my mind and heart," the pope said, in an apparent reference to photos and TV images of victims of chemical weapons in Syria.
"There is the judgment of God, and also the judgment of history, upon our actions," he said, "from which there is no escaping."
Usually soft-spoken, Francis raised his voice as he declared, "War brings on war! Violence brings on violence."
His admonishment against resorting to arms as a solution recalled the repeated emotional implorations a decade ago by the late Pope John Paul II in a vain attempt to persuade the U.S. administration then led by President George W. Bush not to invade Iraq.
The deteriorating drama of Syria inspired Francis to set aside Sept. 7 as a day of fasting and prayer for Syria.
Francis invited Catholics, other Christians, those of other faiths and non-believers who are "men of good will" to join him that evening in St. Peter's Square to invoke the "gift" of peace for Syria, the rest of the Middle East and worldwide where there is conflict.
"The world needs to see gestures of peace and hear words of hope and of peace," Francis said.
He said the prayer vigil in the square will last from 7 p.m. until midnight.