Francis visited the ground zeros of Hiroshima and Nagasaki, both seared in the world's collective consciousness after the bombs dropped there by the United States three days apart in August 1945 in an effort to end World War Two.
"Here, in an incandescent burst of lightning and fire, so many men and women, so many dreams and hopes, disappeared, leaving behind only shadows and silence," Francis said at Hiroshima's Peace Memorial after standing in silent prayer and listening to a harrowing account by a survivor.
Yoshiko Kajimoto, who was 14 at the time, recalled "people walking side by side like ghosts, people whose whole body was so burnt that I could not tell the difference between men and women, their hair standing on end, their faces swollen to double size, their lips hanging loose, with both hands held out with burnt skin hanging from them."
"No one in this world can imagine such a scene of hell," she said.
More than 100,000 people died instantly in the twin attacks and about 400,000 others died in subsequent months, years and decades of radiation sickness or illnesses.
"With deep conviction I wish once more to declare that the use of atomic energy for purposes of war is today, more than ever, a crime not only against the dignity of human beings but against any possible future for our common home," the pope said in Hiroshima. "The use of atomic energy for purposes of war is immoral, so too the possession of nuclear weapons is immoral, as I already said two years ago."
While his words in Hiroshima struck emotional, almost poetic notes, earlier in Nagasaki he issued direct denunciations and demands.
He restated his support for a 2017 treaty to ban nuclear weapons that was agreed by nearly two-thirds of U.N. members but opposed by big nuclear powers who say it could undermine nuclear deterrence, which they credit with averting conventional war.
"Our world is marked by a perverse dichotomy that tries to defend and ensure stability and peace through a sense of security sustained by a mentality of fear and mistrust," he said in a somber voice, amid driving rain and strong wind.
Resources spent on the "arms race" should be used for development and protection of the environment, Francis said in Nagasaki.
"In a world where millions of children and families live in inhumane conditions, the money squandered and the fortunes made through the manufacture, upgrading, maintenance and sale of ever more destructive weapons, are an affront crying out to heaven," he said.
Last August, the United States pulled out of one landmark strategic arms accord, the Intermediate-Range Nuclear Forces Treaty (INF), citing violations by Russia that Moscow denies.
Nuclear experts said it also appeared doubtful that agreement on a full-fledged replacement for the New START nuclear arms control treaty between Russia and the United States will be in place before it expires in February 2021.
In Nagasaki, the pope delivered his appeal standing near a large print of a famous photograph titled "The Boy Standing by the Crematory" taken by an American soldier shortly after the blast. It shows a Japanese boy taking his dead younger brother to be cremated.