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 on Sunday railed against atomic weapons, the nuclear deterrent and the growing arms trade, as he paid tribute to the victims of the "unspeakable horror" of the Nagasaki bomb.

In a highly symbolic visit to the Japanese city devastated by the nuclear attack in August 1945, Francis said nuclear weapons were "not the answer" to a desire for security, peace and stability.

"Indeed they seem always to thwart it," he said.

At least 74,000 people died from the atomic bomb unleashed on the city in western Japan - just three days after the world's first nuclear attack hit Hiroshima and killed at least 140,000.

"This place makes us deeply aware of the pain and horror that we human beings are capable of inflicting upon one another," said the somber pontiff on the first full day of his Japan trip.

Hundreds of people in white waterproofs sat in torrential rain to hear the pope's speech, next to the emblematic photo of a young boy carrying his dead baby brother on his back in the aftermath of the attack.

He laid a wreath of white flowers and prayed silently, unprotected from the lashing downpour.

Francis took aim at what he called the "perverse dichotomy" of nuclear deterrence, saying that peace is incompatible with the "fear of mutual destruction or the threat of total annihilation."

This marked a break with past pontiffs - in a 1982 UN speech, Pope John Paul II had described nuclear deterrence as a necessary evil.

The 82-year-old Francis also hit out at the "money that is squandered and the fortune made" in the arms trade, describing it as an "affront crying out to Heaven" in a world where "millions of children are living in inhumane conditions."

Later Sunday, Francis will visit Hiroshima and meet ­survivors of the atomic attack, known in Japanese as hibakusha, at the world-famous Peace Memorial in the city synonymous with the horror of nuclear war.

Two survivors of Nagasaki, 89-year-old Shigemi Fukahori and 85-year-old Sakue Shimohira, handed the wreath to the pope. 

At a Mass at a baseball stadium in Nagasaki with worshippers now shielding their eyes from the sun, Francis said the city "bears in its soul a wound difficult to heal" and warned that "a third World War is being waged piecemeal."

The Argentine pontiff is fulfilling a long-held ambition to preach in Japan - a country he wanted to visit as a young missionary.

Like in Thailand, the first leg of his Asian tour, Catholicism is a minority religion in Japan. Most people follow a mix of Shinto and Buddhism, with only an estimated 440,000 Catholics in the country.