Pope Francis, a major advocate for nuclear disarmament, is in the atomic-bombed city of Hiroshima.
In a speech, the pope said true peace can only be an unarmed peace. He added the use of nuclear weapons is a crime.
Earlier in the day, the pontiff visited a park in Nagasaki marking the spot where American forces dropped the bomb 74 years ago.
Despite the heavy rain, survivors of the atomic bombing and other invited guests gathered to hear his message of peace. The August 9, 1945 attack on Nagasaki left around 70,000 people dead by the end of that year.
The pope prayed for them twice, bowing deeply at the cenotaph.
He said, "This place makes us deeply aware of the pain and horror that we human beings are capable of inflicting upon one another. The damaged cross and statue of Our Lady recently discovered in the Cathedral of Nagasaki remind us once more of the unspeakable horror suffered in the flesh by the victims of the bombing and their families."
The pontiff also spoke about his commitment to support the international arms control framework, including a treaty prohibiting nuclear weapons.
That treaty has yet to be joined by many countries, such as Japan, which relies on the US nuclear umbrella.
He called on world leaders to commit to the cause.
"Convinced as I am that a world without nuclear weapons is possible and necessary, I ask political leaders not to forget that these weapons cannot protect us from current threats to national and international security. We need to ponder the catastrophic impact of their deployment, especially from a humanitarian and environmental standpoint, and reject heightening a climate of fear, mistrust and hostility fomented by nuclear doctrines."