Pope Francis lights a candle at the Atomic Bomb Hypocenter during a visit to Nagasaki, Japan on Sunday. Pope Francis railed against the use of nuclear weapons and the growing arms trade as he paid tribute to the "unspeakable horror" suffered by victims of the Nagasaki atomic bomb. Photo: AFP

Pope Francis left Japan on Tuesday, bringing to an end a long-desired visit marked by messages on atomic power and weapons as he comforted survivors of nuclear bombings and accidents.

The emotional centerpiece of the pope's Japan trip was a somber visit to Hiroshima and Nagasaki, the only two cities to have been targeted by nuclear weapons, in attacks that killed at least 140,000 and 74,000 people respectively.

He prayed silently in the driving rain at memorials to the dead and heard harrowing testimony from some of the dwindling group of survivors.

Francis launched a powerful diatribe against both the use and possession of nuclear weapons, calling this a "crime" and describing the arms industry as an "affront crying out to heaven."

He comforted survivors, many of whom broke down in tears upon meeting the pontiff, who recalled the "unspeakable horror" they suffered when the bombs were dropped.

"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," said the pope at the Peace Memorial in Hiroshima.

"In barely an instant, everything was devoured by a black hole of destruction and death."

The 82-year-old later took his anti-­nuclear message to Japanese Prime ­Minister Shinzo Abe, urging him and other world leaders to "promote every means of dissuasion" to ensure such a nuclear catastrophe never happens again.

The pope also met survivors of Japan's "triple disaster," the 2011 earthquake, tsunami and Fukushima meltdown that left around 18,500 dead or missing.

He stopped just short of calling for an end to nuclear power, noting that Japanese bishops had appealed for the abolition of atomic power plants in the wake of the Fukushima meltdowns that left wide swathes of northeastern Japan uninhabitable.

The four-day Japan trip came after a visit to Thailand, also a country with only a small proportion of Catholics.

In Thailand, he preached religious tolerance and warned young people against the pitfalls of technology, cautioning them that the distractions of modern life would leave them "empty, weary, alone and disenchanted."