Thousands of residents were evacuated from an area around a nuclear plant north of Tokyo after fears of a radiation leak, but officials said problems with the reactor's cooling system were not at a critical level.
Underscoring grave concerns about the plant, the U.S. air force delivered coolant to the facility, U.S. Secretary of State Hillary Clinton said.
The unfolding disaster in the wake of the 8.9 magnitude earthquake and 10-metre (33-feet) high tsunami prompted offers of help from dozens of countries.
China said rescuers were ready to help with quake relief while President Barack Obama told Japanese Prime Minister Naoto Kan the United State would assist in any way.
Stunning TV footage showed a muddy torrent of water carrying cars and wrecked homes at high speed across farmland near the coastal city of Sendai, home to one million people and which lies 300 km (180 miles) northeast of Tokyo. Ships had been flung onto a harbour wharf, where they lay helplessly on their side.
Japanese politicians pushed for an emergency budget to fund relief efforts after Kan asked them to "save the country", Kyodo news agency reported. Japan is already the most heavily indebted major economy in the world, meaning any funding efforts would be closely scrutinised by financial markets.
Domestic media said the death toll was expected to exceed 1,000, most of whom appeared to have drowned.
The extent of the destruction along a lengthy stretch of coastline suggested the death toll could rise significantly.
Tsunami warnings were issued across the Pacific but were later lifted for some of the most populated countries in the region, including Australia, Taiwan and New Zealand.
Even in a nation accustomed to earthquakes, the devastation was shocking.
"A big area of Sendai city near the coast, is flooded. We are hearing that people who were evacuated are stranded," said Rie Sugimoto, a reporter for NHK television in Sendai.
"About 140 people, including children, were rushed to an elementary school and are on the rooftop but they are surrounded by water and have nowhere else to go."
The quake, the most powerful since Japan started keeping records 140 years ago, sparked at least 80 fires in cities and towns along the coast, Kyodo said.
Other Japanese nuclear power plants and oil refineries were shut down and one refinery was ablaze. Television footage showed an intense fire in the waterfront area near Sendai. There were also reports that an irrigation dam had broken and swept away houses in Fukushima prefecture.
Chief Cabinet Secretary Edano told people to stay in safe places as the cold deepened into the night. "Please help each other and act calmly," he told a news conference.
In Tokyo, residents who had earlier fled swaying buildings jammed the streets trying to make their way home after much of the city's public transportation was halted.
Many subways in Tokyo later resumed operation but trains did not run. People who decided not to walk home slept in office buildings.
"I was unable stay on my feet because of the violent shaking. The aftershocks gave us no reprieve. Then the tsunamis came when we tried to run for cover. It was the strongest quake I experienced," a woman with a baby on her back told television in northern Japan.
The central bank said it would cut short a two-day policy review scheduled for next week to one day on Monday and promised to do its utmost to ensure financial market stability.
Auto plants, electronics factories and refineries shut, roads buckled and power to millions of homes and businesses was knocked out. Several airports, including Tokyo's Narita, were closed and rail services halted. All ports were shut.
The disaster occurred as the world's third-largest economy had been showing signs of reviving from an economic contraction in the final quarter of last year. The disaster raised the prospect of major disruptions for many key businesses and a massive repair bill of billions of dollars.
The tsunami alerts revived memories of the giant waves which struck Asia in 2004. The Pacific Tsunami Warning Center issued alerts for countries to the west and across the Pacific as far away as Colombia and Peru.
The earthquake was the fifth most powerful to hit the world in the past century.
TV footage showed boats, cars and trucks tossed around like toys in the water after a small tsunami hit the town of Kamaichi in northern Japan. An overpass, location unknown, appeared to have collapsed and cars were turning around and speeding away.
"The building shook for what seemed a long time and many people in the newsroom grabbed their helmets and some got under their desks," Reuters correspondent Linda Sieg said in Tokyo. "It was probably the worst I have felt since I came to Japan more than 20 years ago."
The quake struck just before the Tokyo stock market closed, pushing the Nikkei down to end at a five-week low. Nikkei futures trading in Osaka tumbled as much as 4.7 percent in reaction to the news.
The quake surpasses the Great Kanto quake of Sept. 1, 1923, which had a magnitude of 7.9 and killed more than 140,000 people in the Tokyo area.
The 1995 Kobe quake caused $100 billion in damage and was the most expensive natural disaster in history. Economic damage from the 2004 Indian Ocean tsunami was estimated at about $10 billion.
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