Tokyo (CNN) -- The most powerful earthquake to hit Japan in at least 100 years unleashed walls of water Friday that swept across rice fields, engulfing towns, dragging houses onto highways and tossing cars and boats like toys, apparently killing hundreds and forcing the evaucations of tens of thousands.
Prime Minister Naoto Kan said the "enormously powerful" earthquake has caused "tremendous damage over a wide area".
The quake, which struck at 2:46 pm local time, sparked fires in at least 80 locations, Japan's Kyodo News Service reported, and prompted the U.S. National Weather Service to issue tsunami warnings for at least 50 countries and territories.
Police in Miyagi Prefecture say between 200-300 have been found in the coastal city of Sendai alone, Japan's Kyodo News Agency reported late Friday. The death toll is likely to rise as there are few casualty counts yet from the worst-hit areas.
Kyodo, citing Japan's defense forces, said 60,000 to 70,000 people were being evacuated to shelters in the Sendai area.
Japanese authorities ordered the precautionary evacuation of a nuclear plant affected by the earthquake, saying that while there was no immediate danger, crews were having trouble cooling the reactor. The Fukushima plant is one of four closest to the quake that the U.N. nuclear watchdog agency said were safety shut down.
The epicenter was offshore 373 kilometers (231 miles) away from Tokyo, the United States Geological Survey said.
But residents there continued to feel aftershocks hours after the quake. More than 30 aftershocks followed, with the strongest measuring 7.1.
The prime minister said an emergency task force has been activated, and appealed for calm. The government dispatched 8,000 troops to assist in the recovery effort and asked for U.S. military assistance, according to Kyodo.
A spokesman for the U.S. military bases in Japan said all service members were accounted for and there were no reports of damage to installations or ships.
President Barack Obama, while offering his condolences, said the United States was standing by to help "in this time of great trial".
Images from Japanese media and CNN iReporters show smoke pouring from buildings and water rushing across fields carrying away entire structures.
"I wasn't scared when it started ... but it just kept going and going," said Michelle Roberts, who lives in central Tokyo. "I won't lie, it was quite scary. But we are all OK. We live on the third floor, so most everything shook and shifted".
The quake toppled cars off bridges and into waters underneath. Waves of debris flowed like lava across farmland, pushing boats, houses and trailers. About 4 million homes had no power in Tokyo and surrounding areas.
Firefighters battled a fiery blaze at an oil refinery in Chiba prefecture near Tokyo.
The quake disrupted rail service and left people stranded in their offices. It also affected air travel. Hundreds of flights were canceled, Kyodo said. Some 13,000 people were stranded at the Narita airport, with 10,000 stuck at the Haneda airport, Kyodo said.
At Tokyo Station, one of Japan's busiest subway stations, shaken commuters grabbed one another to stay steady as the ground shook. Dazed residents poured into the streets after offices and schools were closed. Children cried.
Residents said though earthquakes are common in Japan, Friday's stunned most people. CNN
