12 March 2011 Last updated at 01:09 GMT
A mammoth relief mission is swinging into action in north-east Japan, a day after it was struck by a devastating tsunami, claiming hundreds of lives.
The disaster was triggered by an 8.9-magnitude earthquake, the country's most powerful since records began.
Japan's military has mobilised thousands of troops, hundreds of planes and dozens of ships.
The government has warned there could be a radioactive leak at a nuclear power reactor shut down by the quake.
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At the scene
image of Roland Buerk Roland Buerk BBC News, Tokyo
In the centre of Tokyo many people are spending the night in their offices. But thousands, perhaps millions, chose to walk home. Train services were suspended.
Even after the most violent earthquake anyone could remember the crowds were orderly and calm. The devastation is further to the north, along the Pacific coast.
There a tsunami triggered by the quake reached 10km (six miles) inland in places carrying houses, buildings, boats and cars with it. In the city of Sendai the police found up to 300 bodies in a single ward. Outside the city in a built-up area a fire blazed across several kilometres.
12 March 2011 Last updated at 01:09 GMT
Japan begins quake relief mission
Japan's ground self-defence forces have been deployed, and the government has asked the US military based in the country for help. The scale of destruction from the biggest quake ever recorded in Japan will become clear only at first light.
The tremor struck in the afternoon local time on Friday at a depth of about 24km, 400km (250 miles) north-east of Tokyo.
It was nearly 8,000 times stronger than the one which devastated Christchurch, New Zealand, last month, scientists said.
About 300 people are known to have died and more than 500 are missing. Japanese media says the death toll will exceed 1,000.
Prime Minister Naoto Kan plans to hold an emergency cabinet meeting early on Saturday, before visiting the disaster zone by helicopter.
The country's military has mobilised thousands of troops, 300 planes and 40 ships for the relief effort.
US President Barack Obama said a US aircraft carrier was already in Japan, and another was on the way.
The quake triggered a tsunami up to 10m (30ft), with waves of 7m battering the Japanese coast.
A muddy torrent of water swept cars and homes far inland, turning residential areas and paddy fields into a lagoon of debris-filled sea water.
One of the worst-hit areas was the port city of Sendai, in Miyagi prefecture, where up to 300 bodies have been found in one ward alone.
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Map
* Japan quake: video reports
* Quake: Wave forecast map
Japan Railways said it could not trace four trains along the north-eastern coast. A ship carrying 100 people was also reported missing.
Swathes of Kesennuma, in Miyagi prefecture, have burned into the night, while one-third of the city was said to be under water.
Some 1,800 homes were reported to have been destroyed in the city of Minamisoma, Fukushima prefecture.
And a dam burst in north-eastern Fukushima prefecture, sweeping away homes, Kyodo news agency reported.
Meanwhile, Japanese authorities declared a state of emergency at five reactors at the Fukushima I and II plants, as cooling systems failed because of the earthquake.
They also warned there could be small radiation leaks as steam was released from the reactors, where pressure was reported to be considerably higher than normal.
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“Start Quote
It was the biggest earthquake I have ever felt, I thought I would die”
End Quote Sayaka Umezawa Student
Thousands of people were being evacuated around the two plants.
More than 50 aftershocks - many of them more than magnitude 6.0 - have rattled the country.
"It was the biggest earthquake I have ever felt. I thought I would die," said Sayaka Umezawa, a 22-year-old student who was visiting the port of Hakodate.
In central Tokyo, a number of office workers have spent the night in their offices because the lifts stopped working.
Millions of commuters were stranded overnight, while others walked home, after train services were suspended.
At least 20 people were injured in Tokyo when the roof of a hall collapsed on to a graduation ceremony.
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Deadliest earthquakes
27 July 1976, Tangshan, China: est 655,000 killed, 7.5
12 Jan 2010, Haiti: 222,570 killed, 7.0
8 Oct 2005, Pakistan: 80,361 killed, 7.6
31 May 1970 Chimbote, Peru: 70,000 killed, 7.9
Source: USGS
* History of deadly earthquakes
* How to measure earthquakes
* Animated guide: Earthquakes
* Animated guide: Tsunamis
About four million homes in and around the city suffered power cuts.
The tsunami rolled across the Pacific at the speed of a jetliner but had weakened before it hit Hawaii and the US West Coast.
Thousands of people were ordered to evacuate coastal areas in the states of California, Oregon and Washington.
A port in Oregon is reported to have been seriously damaged by the waves.
In the immediate aftermath of the quake, a tsunami warning extended across the Pacific to North and South America, where many other coastal regions were evacuated.
But the alert was later lifted in most parts, including the Philippines, Indonesia, Australia and China.
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