Japan was Saturday preparing to mark the first anniversary of its tsunami, as government papers revealed ministers were warned of the possibility of meltdowns at Fukushima just after the waves struck.

A summary of a government meeting held about four hours after a giant earthquake sent a wall of water crashing into the atomic power station showed that one unidentified participant had cautioned of the risk of a meltdown.

"If the temperature of the reactor cores rises after eight hours, there is a possibility that a meltdown will occur," the person said, according to the summary released on Friday.

Fukushima Daiichi, 220 kilometres (140 miles) northeast of Tokyo, spewed radiation after its cooling systems were knocked out by the tsunami when it crushed coastal communities and left more than 19,000 people dead or missing.

The government and plant operator Tokyo Electric Power (TEPCO) maintained for months there had been no meltdown at Fukushima, despite repeated warnings from independent experts.

They finally admitted in May that three of six reactors had suffered meltdowns.

Tens of thousands of people were made homeless by the nuclear crisis and some tracts of land inside a 20-kilometre exclusion zone are expected to be uninhabitable for decades because of radiation levels.

Friday's revelations came as Japan prepared to mark the first anniversary of the tragedy.

Much of public life will pause on Sunday at 2.46 pm (0546 GMT) to mark the exact moment a 9.0 magnitude earthquake struck off the country's east coast on March 11 last year.

Public transport was expected to stop, and Tokyo's busy shopping districts will fall silent for a minute as the country honours the memory of those who died when the towering tsunami smashed ashore.

A formal ceremony in the capital will be the centrepiece of Japan's official remembrance, with speeches from Prime Minister Yoshihiko Noda and Emperor Akihito.

The 78-year-old emperor, who underwent heart surgery three weeks ago, will attend 20 minutes of the hour-long ceremony with Empress Michiko, the royal household said Saturday.

Some trains in and around Tokyo will be stopped to observe the silence on Sunday, Kyodo reported.

Anti-nuclear protests were scheduled in Tokyo and Fukushima, as well as other parts of the country, while candle-lighting ceremonies were expected to commemorate the victims of the natural disaster.

Setsuko Kuroda, who was organising a two-day anti-nuclear protest in Koriyama, near Fukushima, said 20,000 people were expected on Sunday.

"We demand all children are evacuated from Fukushima now," she told AFP.

"Some experts say one-third of children in Fukushima were affected by radiation.

"Leaving the situation like this is like they are committing a murder everyday."

Tatsuko Sekine, who farms in nearby Sukagawa, said radiation fears haunted her.

"We have to continue living with the fear," she said. "I'm concerned that I will suffer cancer in the future."

Junior high school student Koshiro Nagata said the last year had been tough.

"Tomorrow, I will think about what happened on the day a year ago and pray for the victims," he said. "I want to live without forgetting about the year."

Writing in the Washington Post, Noda said the events of March 11 were etched in the nation's memory.

"We will not forget the loved ones, friends and colleagues lost in the disaster," he said.

Noda pledged to press ahead with reconstruction of tsunami-hit areas and with the full decommissioning of Fukushima Daiichi, as well as decontamination of irradiated land and the "revitalisation" of the Japanese economy.

But for some of those who lost loved ones, a more pressing need to lay ghosts to rest was at hand.

In badly-hit Iwate prefecture, home to a third of the more than 3,200 still listed as missing in the disaster, 300 police and 80 coastguard officers on Saturday continued an intensive three-day search for bodies.

It followed the end of a search in Fukushima on Friday, where hundreds of police and volunteers spent two days looking for the bodies of more than 200 people still missing in the prefecture.

Some of the bodies may be inside the no-go zone around the nuclear plant, which has been off-limits for the last year.

In Nihonmatsu, 71-year-old Yoshiko Kanno, who has lived in temporary housing since fleeing her home in nuclear-hit Namie, said on the anniversary she would be praying for the souls of those who died.

"It was a year of fears and anxiety," she said. "I never dreamed of being here a year ago. My wish is that I won't end my life here."