A 33ft wave has hit the port in the country's Sendai city, sending ships crashing into the shore and carrying cars and buildings through the streets of coastal towns.
At least three people have died following the earthquake, which has obliterated Tokyo's Disneyland car park and caused large fires at sites including an oil refinery in nearby Iichihara.
Large aftershocks continue to hit the capital, following an initial tremor so strong it was felt 1,500 miles away in the Chinese capital Beijing.
Japanese prime minister Naoto Kan said there had been no nuclear plant radiation leak caused by the disaster, but airports have been closed and the Shinkansen 'bullet' train stopped.
Millions of homes are without power and Tokyo's fire department reported several people were injured when a roof caved in at a graduation ceremony in the city.
The Pacific Tsunami Warning Center in Hawaii has put in place a tsunami warning for the entire Pacific, including Guam, Taiwan, the Philippines, Indonesia, Hawaii, Australia and South America.
Russia, Marcus Island and the Northern Marianas have also been put on alert.
Waves from the quake are expected to hit the coast of Taiwan at around 9.30am GMT.
In downtown 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 the city.
Police and coast guard officials said they were assessing possible damage from the quake.
Several quakes had hit the same region in recent days, including a 7.3 magnitude one on Wednesday.