In various locations along Japan's coast, TV footage showed severe flooding, with dozens of cars, boats and even buildings being carried along by waters.
A large ship swept away by the tsunami rammed directly into a breakwater in Kesennuma city in Miyagi prefecture, according to footage on public broadcaster NHK.
Officials were trying to assess possible damage from the quake but had no immediate details.
The quake that struck at 2.46pm was followed by a series of 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 8.8.
The meteorological agency issued a tsunami warning for the entire Pacific coast of Japan. National broadcaster NHK was warning those near the coast to get to safer ground.
The Pacific Tsunami Warning Centre in Hawaii said a tsunami warning was in effect for Japan, Russia, Marcus Island and the Northern Marianas.
A tsunami watch has been issued for Guam, Taiwan, the Philippines, Indonesia and the US state of Hawaii.
The quake struck at a depth of six miles, about 80 miles off the eastern coast, the agency said. The area is 240 miles north east of Tokyo.
In central Tokyo, large buildings shook violently and workers poured into the street for safety. TV footage showed a large building on fire and bellowing smoke in the Odaiba district of Tokyo.
In central Tokyo, trains were stopped and passengers walked along the tracks to platforms.
Footage on NHK from their Sendai office showed employees stumbling around and books and papers crashing from desks.
Several quakes had hit the same region in recent days, including a 7.3 magnitude one on Wednesday.
Thirty minutes after the quake, tall buildings were still swaying in Tokyo and mobile phone networks were not working.
Japan's Coast Guard has set up task force and officials are standing by for emergency contingencies, Coast Guard official Yosuke Oi said.
"I'm afraid we'll soon find out about damages, since the quake was so strong," he said.
TV footage shot from aircraft showed massive walls of water crashing into the coast, destroying everything in their path.
The waves reached heights in excess of 30ft, according to reports.
The sea water travelled far inland, carrying tons of debris with it.
Motorists could be seen frantically trying to speed away from the waves.
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