Japan's most powerful earthquake since records began has struck the north-east coast, triggering a massive tsunami.
Cars, ships and buildings were swept away by a wall of water after the 8.9-magnitude tremor, which struck about 400km (250 miles) north-east of Tokyo.
A state of emergency has been declared at a nuclear power plant, where pressure has exceeded normal levels.
Officials say 350 people are dead and about 500 missing, but it is feared the final death toll will be much higher.
In one ward alone in Sendai, a port city in Miyagi prefecture, 200 to 300 bodies were found.
The quake was the fifth-largest in the world since 1900 and nearly 8,000 times stronger than the one which devastated Christchurch, New Zealand, last month, said scientists.
Thousands of people living near the Fukushima nuclear power plant have been ordered to evacuate.
Japanese nuclear officials said pressure inside a boiling water reactor at the plant was running much higher than normal after the cooling system failed.
Officials said they might need to deliberately release some radioactive steam to relieve pressure, but that there would be no health risk.
US Secretary of State Hillary Clinton had earlier said the US Air Force had flown emergency coolant to the site.
But US officials later said no coolant had been handed over because the Japanese had decided to handle the situation themselves.
The UN's nuclear agency said four nuclear power plants had shut down safely.
Measured at 8.9 by the US Geological Survey, it struck at 1446 local time (0546 GMT) at a depth of about 24km.
The tsunami rolled across the Pacific at 800km/h (500mph) - as fast as a jetliner - before hitting Hawaii and the US West Coast, but there were no reports of major damage from those regions.
Thousands of people were ordered to evacuate coastal areas in the states of California, Oregon and Washington.
The biggest waves of more than 6-7ft (about 2m) were recorded near California's Crescent City, said the Pacific Tsunami Warning Centre.
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, Australia and China.
Strong waves hit Japan's Miyagi and Fukushima prefectures, damaging dozens of coastal communities.
A 10m wave struck Sendai, deluging farmland and sweeping cars across the airport's runway. Fires broke out in the centre of the city.
Japan's NHK television showed a massive surge of debris-filled water reaching far inland, consuming houses, cars and ships.
Motorists could be seen trying to speed away from the wall of water.
In other developments:
- Four trains are missing along the coast, says Japan Railways; and a ship carrying 100 people was swept away
- Fire has engulfed swathes of the coast in Miyagi prefecture's Kesennuma city, one-third of which is reportedly under water
- A major explosion hit a petrochemical plant in Sendai; further south a huge blaze swept an oil refinery in Ichihara city, Chiba prefecture
- Some 1,800 homes are reported to have been destroyed in the city of Minamisoma, Fukushima prefecture
- A dam burst in north-eastern Fukushima prefecture, sweeping away homes, Kyodo news agency reports
- At least 20 people were injured in Tokyo when the roof of a hall collapsed on to a graduation ceremony
In a televised address, Prime Minister Naoto Kan extended his sympathy to the victims of the disaster.
As aftershocks rattled the country, residents and workers in Tokyo rushed outside to gather in parks and open spaces.
Many people in the Japanese capital said they had never felt such a powerful earthquake.
In central Tokyo, a number of office workers are spending the night in their offices because the lifts have stopped working.
"This is the kind of earthquake that hits once every 100 years," said restaurant worker Akira Tanaka.
Train services were suspended, stranding millions of commuters in the Japanese capital.
About four million homes in and around Tokyo suffered power cuts.