He urged people of all faiths to join in the 7 September initiative as he made his address to tens of thousands of people, in St Peter’s Square on Sunday.
“War, never again,” Reuters quotes him as saying.
“Violence never leads to peace, war leads to war, violence leads to violence.”
The Pope echoed his views on Twitter on Monday.
He also condemned the use of chemical weapons, Vatican Radio reports.
War never again! Never again war!— Pope Francis (@Pontifex) September 2, 2013
In an apparent reference to Syria, he said: "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.
“There is the judgement of God, and also the judgement of history, upon our actions, from which there is no escaping."
The prayer vigil will take place in the square from 7pm until midnight, the Associated Press said.