A 4-metre tsunami washed away cars and tore away buildings along Japan's eastern coast near the epicentre after the earthquake hit at 2.46pm (6.46pm NZT). There were reports of injuries in Tokyo.
Officials were trying to assess damage, injuries and deaths from the quake but had no immediate details.
The quake that struck was followed by a series of powerful aftershocks, including a 7.4-magnitude one about 30 minutes later. The US Geological Survey upgraded the strength of the first quake to a magnitude 8.9, while Japan's meteorological agency measured it at 7.9.
The meteorological agency issued a tsunami warning for the entire Pacific coast of Japan. NHK was warning those near the coast to get to safer ground.
The quake struck at a depth of 10km, about 125km off the eastern coast, the agency said. The area is 380km northeast of Tokyo.
In downtown Tokyo, large buildings shook violently and workers poured into the street for safety, and TV footage showed fires across the city in buildings and at an oil refinery.
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