Prime Minister Naoto Kan said the situation at the stricken power plant remains grave, and that Japan is facing its worst crisis since the end of World War II -- which left the defeated country in ruins.
"The current situation of the earthquake, tsunami and the nuclear plants is in a way the most severe crisis in the 65 years since World War II," Kan said in a televised national address.
"Whether we Japanese can overcome this crisis depends on each of us," said the premier, who was wearing an emergency services suit.
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Kan said the shutdown of reactors across the quake zone will involve rolling power outages nationwide, and urged citizens to conserve energy. Japan's nuclear industry provides about a third of its power needs.
"There is the possibility of an explosion in the number three reactor," he said, while voicing confidence that it would withstand the blast as the first reactor had the day before.
A meltdown occurs when a reactor core overheats and causes damage to the facility, potentially unleashing radiation into the environment.
Edano said some radiation escaped in the accident, but that the levels released into the air are so far not high enough to affect human health.
A cooling pump at another plant 120km from Tokyo, the Tokai No 2, had failed, but a back-up is working and cooling the reactor, a plant spokesman said early on Monday.
The United Nations said a total of 590,000 people had been evacuated in the quake and tsunami disaster, including 210,000 living near the Fukushima nuclear plants.
The colossal 9.0 magnitude tremor sent waves of churning mud and debris racing over towns and farmland in Japan's northeast, destroying everything in its path and reducing swathes of countryside to a swampy wasteland.
The immense force of the quake moved Honshu -- the main Japanese island -- 2.4 metres, the US Geological Survey said.
In the small port town of Minamisanriku alone about 10,000 people were unaccounted for -- more than half the population of the town, which was practically erased, public broadcaster NHK reported.
The police chief in Miyagi prefecture -- where Minamisanriku is situated -- said the death toll is certain to exceed 10,000 in his region.
But in a rare piece of good news, a man who was swept 15km out to sea along with his house by the tsunami was plucked to safety on Sunday after being spotted clinging to a piece of the roof.
Hiromitsu Shinkawa, 60, was discovered by a Japanese destroyer and transported by helicopter to hospital, where he was in surprisingly good health after his miracle rescue.
But as the world's third-largest economy struggled to assess the full extent of the disaster, groups of hundreds of bodies were being found along the shattered coastline.
Edano said at least 1000 people are believed to have lost their lives, and police said more than 215,000 people are huddled in emergency shelters.
In the city of Fukushima, about 80km northwest of the stricken nuclear plant, people were panic-buying at supermarkets and petrol
stations had run dry on Sunday.
Otomo Miki was with her husband, three children and their 82-year-old grandfather when the quake hit their home of Sendai.
They managed to get to their car and speed to safety before the tsunami roared through.
"I had to keep zig-zagging around people and water to get to safety," she said. "We've lost our house and we have no idea what's going to happen next."
Her older sister was in a bus when the wave about 10 metres high crashed through.
"The bus driver told everybody to get out of the bus and run,"
Miki said. "My sister got out but some people just couldn't run fast enough," she said, adding that they were swept away in the waves.
The sheer power of the water tossed cars like small toys, upturned trucks that now litter the roads, and left shipping containers piled up along the shore.
Many survivors were left without water, electricity, fuel or enough food, as authorities appeared overwhelmed by the monumental
scale of the disaster.
Japan committed 100,000 troops -- about 40 per cent of the armed forces -- to spearhead a mammoth rescue and recovery effort with hundreds of ships, aircraft and vehicles headed to the Pacific coast area.
"There are so many people who are still isolated and waiting for assistance. This reality is very stark," Defence Minister Toshimi Kitazawa said.
The world rallied behind the disaster-stricken nation, with offers of help even from Japan's traditional rival China.
The US aircraft carrier Ronald Reagan reached waters off the northeast coast on Sunday, part of a flotilla sent by Japan's close ally which has nearly 50,000 military personnel in the country.
US Navy helicopters were transporting relief supplies for quake and tsunami survivors.
Pope Benedict XVI hailed the "courageous" Japanese people and called for prayers for the victims.
Japan Meteorological Agency's director of earthquake prediction said there was a 70 per cent chance of a 7-magnitude aftershock -- a shake that could destroy buildings and trigger more tsunamis.
Japan sits on the "Pacific Ring of Fire", and Tokyo is in one of its most dangerous areas, where three continental plates are slowly grinding against each other, building up enormous seismic pressure.
