An earthquake measuring 8.9 on the Richter scale rocked Japan's north-eastern coast today, unleashing a 13ft (4m) tsunami which swept boats, cars, buildings and tons of debris miles inland.
Fires triggered by the quake burned out of control up and down the coast.
At least one person was killed and there were reports of several injuries in Tokyo, hundreds of miles away, where buildings shook violently throughout the main quake and the wave of massive aftershocks that followed.
TV footage showed waves of muddy waters sweeping over farmland near the city of Sendai, carrying buildings, some on fire, inland as cars attempted to drive away.
"This is a rare major quake, and damages could quickly rise by the minute," said Junichi Sawada, an official with Japan's Fire and Disaster Management Agency.
Officials were trying to assess damage, injuries and deaths but had no immediate details. Police said at least one person was killed in a house collapse in Ibaraki prefecture, just north-east of Tokyo.
A large fire erupted at the Cosmo oil refinery in Ichihara city in Chiba prefecture near Tokyo and was burning out of control.
Public broadcaster NHK showed footage of a large ship being swept away by the tsunami and ramming directly into a breakwater in Kesennuma city in Miyagi prefecture. Similar destruction was seen in dozens of communities along the coast.
In various locations along the coast, footage showed massive damage from the tsunami, with cars, boats and even buildings being carried along by water.
The quake struck at 2.46pm local time and was followed by five powerful aftershocks within about an hour, the strongest measuring 7.1.
The US Geological Survey upgraded the strength of the first quake to a magnitude 8.9, while Japan's meteorological agency measured it at 8.4.
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 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 (10km), about 80 miles (125km) off the eastern coast, the agency said. The area is 240 miles (380km) north-east of Tokyo.
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 Tokyo.
In central Tokyo, trains were stopped and passengers walked along the tracks to platforms. NHK said more than 4 million buildings without power in Tokyo and its suburbs.
The ceiling in Kudan Kaikan, a large hall in Tokyo, collapsed, injuring an unknown number of people, NHK said.
Osamu Akiya, 46, was working in Tokyo at his office in a trading company when the quake hit.
It sent bookshelves and computers crashing to the floor, and cracks appeared in the walls.
"I've been through many earthquakes, but I've never felt anything like this," he said. "I don't know if we'll be able to get home tonight."
Footage on NHK from its Sendai office showed employees stumbling around and books and papers crashing from desks.
It also showed a glass shelter at a bus stop in Tokyo destroyed by the quake and a weeping woman nearby being comforted by another woman.
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 a 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.
The tsunami roared over embankments in Sendai city, washing cars, houses and farm equipment inland before reversing directions and carrying them out to sea. Flames shot from some of the houses, probably because of burst gas pipes.
In Tokyo, hundreds of people were evacuated from Shinjuku station, the world's busiest, to a nearby park. Trains were halted.
Tokyo's main airport was closed. A large section of the ceiling at the one-year-old airport at Ibaraki, about 50 miles (80km) north-east of Tokyo, fell to the floor with a powerful crash.
TV announcers urged viewers near the shore to move to strong concrete buildings and stay above the third floor.
Japan's Fire and Disaster Management Agency said it was still assessing the damage but had not confirmed any deaths.
One person was injured at a baseball stadium in Sendai, but his condition was not immediately known.
Dozens of fires were reported in northern prefectures of Fukushima, Sendai, Iwate and Ibaraki. Houses collapsing and landslides were also reported in Miyagi.