Pope Paul VI appealed for disarmament and peace in a historic visit to the United Nations (UN).
The head of the Roman Catholic Church was the first pope to address the UN General Assembly in New York.
He was also the first pontiff to visit the United States.
He urged the world's representatives at the UN: "No more war - war never again. Drop your weapons. One cannot love with offensive weapons in place."
He also met American President Lyndon B. Johnson and both expressed the desire to work for peace, given the war in Vietnam and India-Pakistan conflicts, among other hostilities.
Millions of Americans lined the streets of New York to welcome the Pope, who also celebrated a mass for peace at the Yankee Stadium.
He told the Americans: "The first Pope to set foot on your land blesses you with all his heart."
In his day-long visit to New York, he used the word peace over 50 times.
It was estimated that at least 100 million people in the US watched the event on television.