The earthquake which rocked Japan today was 8,000 times more powerful than the one that devastated Christchurch in New Zealand last month, experts said.
The quake, measuring 8.9 on the Richter scale, unleashed a huge tsunami that crashed into Japan's eastern coastline, sweeping boats, cars, buildings and tons of debris miles inland.
Television pictures showed walls of water over 13ft high swamping vast areas of low-lying farmland with motorists racing to escape the deluge.
In downtown Tokyo - hundreds of kilometres away from the quake - large buildings shook violently and workers poured into the streets for safety.
The quake struck at 2.46pm local time (0546 GMT) and was followed by 12 powerful aftershocks, seven of them at least 6.3 on the Richter scale, the size of the quake which struck New Zealand on February 22.
A tsunami alert was issued covering a vast area of the globe, including areas as far apart as New Zealand, Latin American and eastern Russia.
The quake struck at a depth of 10km, about 125km off the eastern coast, Japan's meteorological agency said. The area is 380km north east of Tokyo.
Japan's Prime Minister Naoto Kan said the quake caused "major damage in broad areas" but nuclear power plants in the area were not affected.
The government was preparing to send troops to the quake-hit areas to help relief efforts.
Sendai airport, north of Tokyo, was inundated with cars, trucks and buses and thick mud covered its runways.
A large fire erupted at the Cosmo oil refinery in Ichihara city near Tokyo and was burning out of control with 30m flames whipping into the sky.
Brian Baptie, a seismologist from the British Geological Survey in Edinburgh, said: "This is a earthquake measuring 8.9 on the Richter scale.
"To put that into some sort of context, it's 8,000 times larger than the one that destroyed Christchurch last month, and on a similar scale to the Chile earthquake in February last year.
"An earthquake of this scale is capable of causing huge damage and destruction, and a tsunami that high will cause complete devastation."
The seismologist said it was difficult to judge the scale of the tsunami, but he said this earthquake was smaller than the one that struck off northern Sumatra in 2004, a magnitude 9.3 quake which set off the Boxing Day tsunami, killing thousands of people.