The death toll from Japan’s most powerful earthquake in centuries could top 10,000, national broadcaster NHK quoted a police official as saying.
The massive 8.9-magnitude quake hit the northeast of the country at 2.46pm local time on Friday, 373km from Tokyo, while strong aftershocks also hit northwestern Japan. It triggered fires, deadly 10-metre waves and tsunami alerts across at least 20 countries.
The quake was 8,000 times more powerful than the one which devastated the New Zealand city of Christchurch last month.
Foreign countries have started to send disaster relief teams to help Japan, with the United Nations sending a group to help coordinate work.
The country is struggling to avert a nuclear disaster and care for millions of people without power or water.
Whole villages and towns wiped off the map by a wall of water, leaving in its wake an international humanitarian effort of epic proportion.
A grim-faced Prime Minister Naoto Kan described the crisis at Japan's worst since 1945, as officials confirmed that three nuclear reactors were at risk of overheating, raising fears of an uncontrolled radiation leak.
Officials are working desperately to stop fuel rods in the damaged reactors from overheating. If they fail, the containers that house the core could melt, or even explode, releasing radioactive material into the atmosphere.
The most urgent crisis centres on the Fukushima Daiichi complex, where all three reactors are threatening to overheat, and where authorities say they have been forced to release radioactive steam into the air to relieve reactor pressure.
The complex, 240 km north of Tokyo, was rocked by an explosion yesterday, which blew the roof off a reactor building. The government did not rule out further blasts there but said this would not necessarily damage the reactor vessels.
Authorities have poured sea water in all three of the complex's reactor to cool them down.
The complex, run by Tokyo Electric Power Co., is the biggest nuclear concern but not the only one: the U.N. nuclear watchdog said Japanese authorities had notified it of an emergency at another plant further north, at Onagawa.
But Japan's nuclear safety agency denied problems at the Onagawa plant, run by Tohoku Electric Power Co., noting that radioactive releases from the Fukushima Daiichi complex had been detected at Onagawa, but that these were within safe levels at a tiny fraction of the radiation received in an x-ray.
Shortly afterwards, a cooling-system problem was reported at another nuclear plant closer to Tokyo, in Ibaraki prefecture.
Fukushima's No. 1 reactor, where the roof was ripped off, is 40 years old and was originally set to go out of commission in February but had its operating licence extended by 10 years.
Prime Minister Kan said the crisis was not another Chernobyl, referring to the nuclear disaster of 1986 in Soviet Ukraine.
"Radiation has been released in the air, but there are no reports that a large amount was released," Jiji news agency quoted him as saying. "This is fundamentally different from the Chernobyl accident."
Almost 2 million households are without power in the freezing north, the government said. There were about 1.4 million without running water. Kyodo news agency said about 300,000 people were evacuated nationwide.
Authorities have set up a 20km exclusion zone around the Fukushima Daiichi plant and a 10 km zone around another nuclear facility close by.
The nuclear accident, the worst since Chernobyl, sparked criticism that authorities were ill-prepared for such a massive quake and the threat that could pose to the country's nuclear power industry.
Chief Cabinet Secretary Yukio Edano said there might have been a partial meltdown of the fuel rods at the No. 1 reactor at Fukushima. Engineers were pumping in seawater, trying to prevent the same happening at the No. 3 reactor, he said in apparent acknowledgement they had moved too slowly yesterday.
"Unlike the No.1 reactor, we ventilated and injected water at an early stage," Edano told a news briefing.
The No. 3 reactor uses a mixed-oxide fuel which contains plutonium, but plant operator Tokyo Electric Power Co (TEPCO) said it did not present unusual problems.
TEPCO said radiation levels around the Fukushima Daiichi plant had risen above the safety limit but that it did not mean an "immediate threat" to human health.
The wind over the plant would continue blowing from the south, which could affect residents north of the facility, an official at Japan's Meteorological Agency said.
Mr Kan said food, water and other necessities such as blankets were being delivered by vehicles but because of damage to roads, authorities were considering air and sea transport. He also said the government was preparing to double the number of troops mobilised to 100,000.
Thousands spent another freezing night huddled in blankets over heaters in emergency shelters along the northeastern coast, a scene of devastation after the quake sent a 10-metre wave surging through towns and cities in the Miyagi region, including its main coastal city of Sendai.
A Japanese official said 22 people have been confirmed to have suffered radiation contamination and up to 190 may have been exposed. Workers in protective clothing used handheld scanners to check people arriving at evacuation centres.
The government, in power less than two years and which had already been struggling to push policy through a deeply divided parliament, came under criticism for its handling of the disaster.
"Crisis management is incoherent," blared a headline in the Asahi newspaper, saying information and instructions to expand the evacuation area around the troubled plant were too slow.
There has been a proposal of an extra budget to help pay for the huge cost of recovery.
Agencies