Japan fell silent Sunday to honour the 19,000 people killed a year ago in a catastrophic earthquake and tsunami that triggered the world's worst nuclear crisis in a generation.

Tearful families gathered in towns and villages across the country's shattered northeast to remember those they lost as the towering waves smashed ashore.

At 2.46 pm (0546 GMT) the nation paused for a minute's silence to mark the moment nature's fury was visited on Japan, when the 9.0-magnitude quake set off a devastating chain of events.

At a national ceremony of remembrance in Tokyo silent prayers were led by Japan's prime minister and Emperor Akihito, who said the country would "never forget" its worst post-war calamity.

"Many difficulties lie ahead in the reconstruction of the disaster-affected areas," he said, urging citizens to "join their hearts with the people affected by the disasters, and continue to help them to improve their lives".

A single pillar symbolising the souls of those who died stood in the middle of the stage, decorated with white chrysanthemums and lilies.

Anti-nuclear demonstrations were held across the northeast region where an estimated 160,000 people were forced to evacuate after the monster waves triggered a meltdown at the Fukushima Daiichi nuclear plant.

One year on, more than 340,000 people are still living in temporary housing -- most lost their homes when a wall of black water crushed whole communities.

Others were forced to flee homes in the shadow of Fukushima as it began venting toxic radiation over homes and farmland when its cooling systems were knocked out, unleashing the worst nuclear disaster since Chernobyl.

Many parts of a 20-kilometre (12-mile) exclusion zone imposed around the Fukushima plant are likely to remain uninhabitable for years, perhaps decades.

In Okuma, home to the crippled plant, displaced residents wore anti-radiation suits, gloves and masks on a rare visit to their contaminated home town, where they remembered those they had lost.

An elderly woman, whose grandchild is still listed as missing, wept as she laid flowers at a makeshift altar.

"I want my grandchild to be found," she told reporters.

In the nearby city of Koriyama, around 16,000 people rallied to demand an end to nuclear power in Japan. "Fukushima is being forgotten day by day," said one protester, Yumiko Ono, a 34-year-old graphic designer from Tokyo.

"If we don't raise our voices right now, another accident could happen. We want to tell the world that the crisis and the hardship is still going on."

As darkness fell, candlelight ceremonies were held across the country to wrap up the day of commemoration. In Fukushima city, more than 300 people gathered for "Candle Night" in front of local government offices.

The battle for control of three runaway reactors was declared won in December when the government and plant operator Tokyo Electric Power (TEPCO) announced they were in "a state of cold shutdown".

On Sunday TEPCO President Toshio Nishizawa offered the latest in a line of apologies from a company that has become a byword for mistrust in Japan.

"We sincerely apologise to people in the neighbouring area, in Fukushima and in greater society for... the accident," he said.

The crisis at Fukushima has badly dented the country's faith in atomic power.

Energy-hungry Japan is just two reactors away from total nuclear shut-down, with public disquiet preventing utilities from re-starting shuttered plants amid fears for safety in an earthquake-prone nation.

Swathes of the tsunami-hit coast remain in ruins, with reconstruction work stalled amid arguments over where and how to rebuild. Once tight-knit villages have been scattered and the economy has taken a battering.

In Ishinomaki, home to a fifth of those who died in the disaster, tsunami warning sirens wailed to mark the moment the quake hit, sending huge waves into the city where they claimed the lives of nearly 4,000 people.

"It's been a year since my father died. I am going to pray that I can get over my grief and that my children can feel better," said Hitomi Oikawa, 37.

Speaking at the national ceremony, Prime Minister Yoshihiko Noda pledged Japan would recover from its tragedy.

"Our forebears, who led our country to prosperity, stood up with brave resolution in times of crisis," he said.

"While offering our support for the daily struggles of those people in the disaster-affected regions, we will join hands as we seek to fulfil our historic mission of 'the rebirth of Japan through reconstruction'."