Editor's Note: Read live blogging of the Japan tsunami and earthquake. Are you there? Send your video, pictures to iReport. For more news visit CNN affiliates KHON and KHNL.Tokyo (CNN) -- The most powerful earthquake to hit Japan in recorded history struck off the island nation's shore on Friday, collapsing buildings, touching off widespread fires and unleashing walls of water up to 30 feet high.

The waves swept across rice fields, engulfed towns, dragged houses onto highways, and tossed cars and boats like toys. It reaching as far as about six miles (10 kilometers) inland in Miyagi Prefecture on Japan's east coast.

Hundreds of people were dead and hundreds more missing, Japanese media reported, citing local and national police. Tens of thousands of people were displaced, according to Japan's Kyodo News agency.

Prime Minister Naoto Kan said the "enormously powerful" earthquake had caused "tremendous damage over a wide area."

The 8.9-magnitude 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.

But Japanese government officials said large tsunami waves are still a risk to coastal Japan, and urged residents in coastal areas to move to higher ground.

The epicenter was offshore of Miyagi Prefecture, about 230 miles (370 kilometers) from Tokyo, the U.S. Geological Survey said.

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Residents there continued to feel aftershocks hours after the quake. More than 30 aftershocks followed, with the strongest measuring 7.1.

Japanese broadcasters reported collapsed buildings, power outages and transportation disruptions throughout Japan. In Tokyo, 230 miles from the hardest hit areas, rail service was suspended, elevated highways were shut down early Saturday and surface streets remained jammed as commuters continued trying to get to their homes in outlying areas.

Video aired by Japanese broadcaster NHK showed extensive fires in Miyagi and widespread fires in the port city of 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 News 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.

And a dam in Fukushima Prefecture failed, washing away homes, Kyodo reported. There was no immediate word of casualties, but the Defense Ministry said 1,800 homes were destroyed.

Japan's trade minister, Banri Kaieda, said a small radiation leak could occur at the Fukushima nuclear plant, Japan's Kyodo News Agency reported Saturday.

Prime Minister Kan is planning to inspect the plant, according to Chief Cabinet Secretary Yukio Edano.

The government had ordered the evacuation of residents nearest the plant as efforts to keep it cool after it was shut down were initially hampered.

The official death toll stood at 151, with 539 injured and 351 missing, according to Kyodo, citing police, but that death toll seemed almost certain to rise -- from 200 to 300 bodies have been found in the coastal city of Sendai alone, said NHK, citing police who said the victims may have been struck by a massive tsunami.

Kyodo said the death toll is likely to surpass 1,000.

The news agency, citing Japan's defense forces, also said 60,000 to 70,000 people were being evacuated to shelters in the Sendai area of Miyagi Prefecture.

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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."

The U.S. Navy has begun reconnaissance flights to map the disaster zone and was moving the aircraft carrier USS Ronald Reagan into position to assist the Japanese government with relief efforts, defense officials said.

Two search and rescue teams, totaling more than 140 people, are also on the way, the U.S. Agency for International Development said.

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 also disrupted rail service and 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, the news agency 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 and offices and schools were closed. Children cried.

Residents said that although earthquakes are common in Japan, Friday's stunned most people.

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"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 -- 15.2 miles (24.4 kilometers) -- creates a lot of energy, said Shenza Chen of the U.S. Geological Survey.

As Japan grappled with the devastation, a tsunami generated by the quake 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 in a warning to 50 countries and territories it said could be affected.

But in places where the waves had reached outside Japan, including Guam and Hawaii, officials reported no damage or injuries.

The tsunami brought waves of nearly 7 feet to a harbor in Maui, authorities said, but other areas reported lower levels.

On the U.S. mainland, waves heights from Alaska to California ranged from under a foot to over 8 feet. The highest measurement, 8.1 feet, was at Crescent City, California.

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.

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 quake was the latest in a series around Japan this week.

On Wednesday, a 7.2-magnitude earthquake struck off the coast of Honshu, the country's meteorological agency said. Early Thursday, an earthquake with a preliminary magnitude of 6.3 struck off the same coast.

Friday's quake is the strongest earthquake in recorded history to hit Japan, according to U.S. Geologic Survey records. The previous record was an 8.6 magnitude earthquake that struck near the Chubu Region near southwestern Honshu on October 28, 1707, that may have killed 5,000 people, said CNN meteorologist Sean Morris.

That quake generated a 33-foot (10-meter) tsunami wave, and some scientists believe the quake may have triggered the eruption of Mount Fuji 49 days later, Morris said.

The world's largest recorded quake took place in Chile on May 22, 1960, with a magnitude of 9.5, the USGS said.