ROME, March 5 — Pope John Paul II made several emphatic appeals today for a peaceful resolution to the conflict with Iraq, seizing the occasion of Ash Wednesday to urge world leaders and Roman Catholics everywhere to do what they could to prevent war.

On a holy day for many Christians that coincided with a meeting in Washington between a papal envoy and President Bush, the pope used two public appearances to underscore his opposition to an American-led invasion of Iraq.

"I believe that when peace is at stake, it is never too late for dialogue," the pope said this morning in his weekly appearance before a general audience in St. Peter's Square in Vatican City.

"Everyone has to knowingly assume his responsibility and make a common effort to spare humanity another dramatic conflict," he said, speaking in Italian.