Pope John Paul has called for a day of peace on March 5.
"The future of humanity should never be tied down by terrorism and the logic of war," he said during his regular Sunday address.
"Never, never, never!" he reiterated forcefully from his window overlooking pilgrims gathered in St. Peter's Square.
On Saturday, Pope John Paul II has urged Tony Blair to make "every effort" to avoid going to war with Iraq.
The UK prime minister had a private audience with the 82-year-old pontiff, a staunch opponent of conflict, at the Vatican on Saturday.
In a statement issued after the 15-minute meeting, the Vatican said: "The Holy Father hoped that, in finding a solution to the grave situation in Iraq, every effort must be made to avoid new divisions in the world."