The U.S. dropped an atomic bomb on the city, killing more than 70,000 people and prompting Japan's Second World War surrender.
The first atomic bomb in Hiroshima three days earlier killed 140,000 people.
At his Sunday Angelus Pope Francis said the only way to win a war was 'not to make one'.
He described the bombing as a "horror" and prayed for those who died during the attack.
The atomic bombings of Hiroshima and Nagasaki were symbols of the destructive power of human beings, he said.
"No to war and violence, yes to dialogue and peace," he added.
In Japan survivors of the bombing gathered for a special service.
They attended the annual commemoration in the southern Japanese city along with international guests and others.
They observed a moment of silence at 11.02am local time, when the bomb was dropped.
The Japanese Prime Minister echoed the Pope as he maintained his country's commitment to be nuclear free.
Shinzo Abe said Japan, as the only country to experience nuclear attacks, would seek to play a leading role in realising a world without such weapons.
A message from UN Secretary General Ban Ki-moon also urged the abolition of nuclear weapons.
"I wholeheartedly join you in sounding a global rallying cry: No more Nagasakis. No more Hiroshimas," Mr Ban said in a message read by Kim Won-soo, the acting UN high representative for disarmament affairs.
Elsewhere hundreds of Christians gathered at the Urakami Cathedral in Nagasaki, which was rebuilt after the attack, to pray for the dead.