Tokyo (CNN) -- The most powerful earthquake to hit Japan in at least 100 years unleashed walls of water Friday that swept across rice fields, engulfed towns, dragged houses onto highways and tossed cars and boats like toys, apparently killing hundreds and forcing the evacuations of tens of thousands.
Prime Minister Naoto Kan said the "enormously powerful" earthquake had caused "tremendous damage over a wide area."
Japanese media, citing local and national police, said hundreds of people were dead and hundreds more missing. The 2:46 p.m. quake struck 373 kilometers (230 miles) from Tokyo, the United States Geological Survey said.
The quake prompted the U.S. National Weather Service to issue tsunami warnings for at least 50 countries and territories, although initial reports as the waves reached locations outside of Japan indicated no damage.
Japan's Kyodo News Service, 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 the area around 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.
Video aired by NHK showed widespread fires in Hakodate in the southern part of Hokkaido island in northern Japan. An oil refinery was burning in Chiba Prefecture near Tokyo, according to NHK, and firefighters could not get close enough to fight it because of the heat. And Kyodo said fires could be seen in extensive areas of Kesennuma in Miyagi Prefecture.
Also in Miyagi, officials reported that a train had derailed and authorities had lost contact with another train, Kyodo reported.
The epicenter was offshore about 370 kilometers (230 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 had 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 offered his condolences and said the United States is standing by to help "in this time of great trial."
Images from Japanese media and CNN iReporters showed 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.
The quake disrupted rail service, which was halted for a time following the quake. It also affected air travel. Hundreds of flights were canceled, Kyodo said. Some 13,000 people were stranded at the Narita airport, and 10,000 were stuck at the Haneda airport, Kyodo said.
At Tokyo Station, one of Japan's busiest subway terminals, 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.
"This was larger than anyone expected and went on longer than anyone expected," said Matt Alt, who lives in Tokyo.
"My wife was the calm one. ... She told us to get down and put your back on something, and leave the windows and doors open in case a building shifts so you don't get trapped."
Richard Lloyd Parry said he looked through a window and saw buildings shaking from side to side.
"Central Tokyo is fine from what we see, people are calm ... and not going inside buildings," he said.
Such a large earthquake at such a shallow depth -- 24.4 kilometers (15.2 miles) -- creates a lot of energy, said Shenza Chen of the U.S. Geological Survey.
As the city grappled with the devastation, a massive tsunami swept across the Pacific Ocean.
An earthquake of that size can send a dangerous tsunami to coasts outside the source region, the National Weather Service said.
The National Weather Service's list of countries and territories the could be hit by a tsunami included Russia and Indonesia, Central American countries like Guatemala, El Salvador and Costa Rica, and the U.S. state of Hawaii, where warning sirens were sounded in the morning. A tsunami warning was also issued for areas along the United States and Canadian west coasts.
While some officials feared that waves from the tsunami could be high enough to wash over entire islands in the Pacific, at least one expert said it was unlikely.
The tsunami could cause significant damage and flooding, but "washing over islands is not going to happen," said Gerard Fryer of the Pacific Tsunami Warning Center.
Humanitarian agencies were working with rescue crews to reach the people affected by the earthquake and tsunami.
"When such an earthquake impacts a developed country like Japan, our concern also turns to countries like the Philippines and Indonesia, which might not have the same resources," said Rachel Wolff, a spokeswoman for World Vision.
Wolff said her agency is helping people in Japan and teaming up to help others in countries along the path of the tsunami.
The tsunami could cause damage "along coastlines of all islands in the state of Hawaii," warned the U.S. National Oceanic and Atmospheric Administration. "Urgent action should be taken to protect lives and property."
Tsunamis are a series of long ocean waves that can last five to 15 minutes and cause extensive flooding in coastal areas. A succession of waves can hit -- often the highest not being the first, said CNN meteorologist Ivan Cabrera.
The quake was the latest in a series around Japan this week.
Early Thursday, an earthquake with a preliminary magnitude of 6.3 struck off the coast of Honshu. A day earlier, a 7.2-magnitude earthquake struck off the same coast, the country's meteorological agency said.
The world's largest recorded quake took place in Chile on May 22, 1960, with a magnitude of 9.5, the USGS said.
The quake Friday was the fifth-strongest in the world since 1900, the agency said and the most powerful to hit Japan since then.
CNN's Kyung Lah, Faith Karimi, Ed Payne, Catherine E. Shoichet and Kevin Voigt contributed to this report.
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