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The Japanese government estimates more than 46,000 homes and buildings have been damaged by the devastating earthquake and tsunami that hit northeastern Japan on Friday.
And it says 5,700 of those structures collapsed in the 9.0 magnitude quake or were washed away by the tsunami.
The United Nations says a total of 590,000 people had been evacuated in the quake and tsunami disaster, including 210,000 living near the Fukushima nuclear plants.
Worst crisis since WWII
The country is also battling a feared meltdown of two reactors at a quake-hit nuclear plant, as the full horror began emerging of the disaster on the ravaged northeast coast where more than 10,000 were feared dead.
Prime Minister Naoto Kan said the situation at the stricken Fukushima Daiichi power plant remained grave, and that Japan was 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."
Kan said the shutdown of reactors across the quake zone would entail rolling power outages nationwide, and urged citizens to conserve energy. Japan's nuclear industry provides about a third of its power needs.
While four nuclear plants in northeastern Japan have reported damage, the danger appeared to be greatest at the Fukushima Daiichi nuclear complex, where one explosion occurred Saturday and a second was feared. Operators have lost the ability to cool three reactors at Daiichi and three more at another nearby complex using usual procedures, after the quake knocked out power and the tsunami swamped backup generators.
Chief Cabinet Secretary Yukio Edano said overnight that a hydrogen explosion could occur at Daiichi's Unit 3, the latest reactor to face a possible meltdown. That would follow a hydrogen blast Saturday in the plant's Unit 1.
"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said. "If there is an explosion, however, there would be no significant impact on human health."
Operators have been dumping seawater into units 1 and 3 in a last-ditch measure to cool the reactors. They were getting water into the other four reactors with cooling problems without resorting to corrosive sea water, which likely makes the reactors unusable.
Edano said residents within about 20 kilometres of the Daiichi plant were ordered to evacuate as a precaution, and the radioactivity released into the environment so far was so small it didn't pose any health threats.
Up to 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan's nuclear agency.
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.
In the city of Fukushima, about 80 kilometres northwest of the Daiichi nuclear plant, people were panic-buying at supermarkets and petrol stations had run dry.
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 some 10 metres (more than 30 feet) 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 lorries 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.
Disaster could cost over up to $45b
With ports, airports, highways and manufacturing plants shut down, the government predicted "considerable impact on a wide range of our country's economic activities".
Leading risk analysis firm AIR Worldwide said the quake alone would exact an economic toll estimated at between $NZ20 billion and $NZ45 billion, without taking into account the effects of the tsunami.
Tokyo's benchmark Nikkei index is expected to tumble later today with the benchmark index possibly breaking the psychologically important 10,000 level.
The Bank of Japan plans to pump "massive" funds into markets in a bid to help them stabilise following the quake, Dow Jones Newswires said.
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," said Defence Minister Toshimi Kitazawa.
World rallies behind Japan
The world has 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 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 within the next three days - 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.