The pope arrived in Japan late Saturday on the second leg of an Asia tour that began in Thailand, and his four-day visit is expected to focus heavily on the theme of peace and religious tolerance.

Even before his arrival in Japan, the Argentine pontiff railed in a video message to the Japanese people against the "immoral" use of nuclear weapons.

"Together with you, I pray that the destructive power of nuclear weapons will never be unleashed again in human history," said the head of the world's 1.3 billion Catholics.

He will begin his day in Nagasaki, where he is expected to evoke the horrific destruction and human toll of the twin bomb attacks in a call for a world free of nuclear weapons.

At least 74,000 people were killed in Hiroshima, and 140,000 people in Nagasaki when the United States dropped nuclear weapons on them at the end of World War II.

Francis will deliver his remarks near the site of the attack in Nagasaki and later visit Hiroshima for an address at the world-famous peace memorial.

- 'I never forgot the sight' -

Among the audience will be some survivors of the attack, known in Japanese as hibakusha, as well as relatives of those killed in the devastating bombings.

Minoru Moriuchi was just eight when the bomb was dropped on Nagasaki, and he still recalls the horror of finding his aunt and two cousins dead in the aftermath.

"I never forgot this sight -- their bodies were reddish black and completely burnt," he told AFP.

Like many hibakusha, the 82-year-old fears that the lessons of the Nagasaki and Hiroshima attacks risk being lost when the last generation of survivors dies.

"The Pope never meddles with politics, but I hope people listening to his message will think seriously about the nuclear issue," he said.

Shigeru Tokuyasu, a former minister at the Japanese embassy to the Holy See, said the pope wields an almost unique ability to focus international attention on an issue like nuclear weapons.

"He has the ability to reach out to people across the world," he told AFP before the pope's arrival.

"I don't think his visit will immediately help reduce nuclear arms, but it will be a step forward. It will help change the 'globalisation of indifference'."

- 'Hidden Christians' -

In Nagasaki the pope will also deliver a mass at the city's baseball stadium and offer a message of religious tolerance in a country where Christianity is very much a minority faith.

Most Japanese practise a mixture of Shinto and Buddhism, two closely intertwined faiths based on the worship of nature and spirits, though Christian festivals like Christmas are also popular.

There are only approximately 440,000 Catholics out of a population of 126 million, and Christians endured centuries of bloody repression in Japan after the faith was introduced by missionaries.

In the 17th century, Japan was closed to the outside world and Christians were persecuted and killed if they failed to recant their faith.

But a small community kept the religion alive, creating a unique version of Catholicism blended with Japanese culture and religious rites that was discovered when the country reopened to the outside world in the mid-19th century.

Francis will pay tribute to these so-called "hidden Christians" -- or "kakure kirishitan" in Japanese -- at a memorial to some of the community's "martyrs" in Nagasaki.

He returns to Tokyo on Sunday night where he will on Monday meet victims of Japan's "triple disaster" -- the 2011 earthquake, tsunami and nuclear meltdown.

He is also scheduled to deliver a mass at a Tokyo baseball stadium, meet Japan's new Emperor Naruhito and hold talks with Japanese government officials and local Catholic leaders.

On the first stop of his tour in Thailand, he preached a message of religious tolerance, meeting the country's King Vajiralongkorn and the Buddhist Supreme Patriarch, who leads Thailand's Buddhists.