More than 1,000 people may have died in the massive earthquake and tsunami disaster that devastated parts of Japan on Friday, Kyodo News agency said.
The strongest quake on record unleashed a terrifying 10-metre tsunami, with a nuclear plant and petrochemical complex among multiple sites set ablaze.
The monster wall of water generated by the 8.9-magnitude earthquake -- the seventh biggest in history -- pulverised the northeastern city of Sendai, where police reportedly said that 200-300 bodies had been found on the coast.
The 10-metre (33-foot) wave of black water sent shipping containers, cars and debris crashing through the streets of Sendai and across open farmland, while a tidal wave of debris-littered mud destroyed everything in its path.
The National Police Agency said 137 people had been confirmed dead and 531 missing, with 627 others injured in the tremor, and a spokesman said this did not include the bodies reportedly found on the Sendai coast.
"The damage is so enormous that it will take us much time to gather data," an official at the agency told AFP.
Japan's military mobilised thousands of troops, 300 planes and 40 ships for the relief effort.
An armada of 20 naval destroyers and other ships was headed for the devastated Pacific coast area of Honshu island, while air force jets flew reconnaissance missions over the disaster zone.
The wave set off tsunami alerts across the Pacific, as far away as South America, New Zealand and Hawaii, which was hit by a surge of up to 1.8 metres (six feet) after authorities evacuated low-lying areas. California and Oregon were also on alert, with the Los Angeles county fire captain warning a surge could impact the coastline.
A Japanese ship with 100 people aboard was reportedly carried away, more than 300 houses were destroyed in the remote city of Ofunato and a dam broke in the northeast prefecture of Fukushima, with homes washed away.
"It was the biggest earthquake I have ever felt. I thought I would die," said Sayaka Umezawa, a 22-year-old college student who was visiting the port of Hakodate, which was hit by a two-metre wave.
The government said the tsunami and quake, which was felt in Beijing some 2,500 kilometres (1,500 miles) away, had caused "tremendous damage", while aerial footage showed massive flooding in northern towns.
Prime Minister Naoto Kan, wearing an emergency services suit in a national television address, said he had established an emergency headquarters for disaster response and called for calm from the public.
The quake, which hit at 2:46 pm (0546 GMT) and lasted about two minutes, rattled buildings in greater Tokyo, the world's largest urban area and home to some 30 million people.
Millions who had earlier fled swaying buildings in the capital were left stranded after the earthquake shut down the city's vast subway system. The mobile phone network was strained to breaking point.
The government used loudspeaker alerts and TV broadcasts to urge people to stay near their workplaces rather than risk a long walk home, as highways leading out of the city centre were choked and hotels rapidly filled up.
There was also major disruption to air travel and bullet train services. A passenger train with an unknown number of people aboard was unaccounted for on a line outside Sendai, Kyodo News reported.
The government insisted there was no risk of radiation leaking from Japan's network of advanced nuclear power plants, which are designed to shut down as soon as the earth shakes in one of the world's most quake-prone countries.
Authorities ordered 2,000 residents living by a nuclear plant in Fukushima to evacuate after a reactor cooling system failed, though Jiji Press later reported the system was expected to return to normal.
A fire broke out in the turbine building of another nuclear plant in Onagawa.
The tsunami also reached Sendai airport, submerging the runway while a process known as liquefaction, caused by the intense shaking of the tremor, turned parts of the ground to liquid.
Plumes of smoke rose from at least 10 locations in Tokyo, where four million homes suffered power outages.
Hours after the quake struck, TV images showed huge orange balls of flame rolling up into the night sky as fires raged around a petrochemical complex in Sendai.
A massive fire also engulfed an oil refinery in Iichihara near Tokyo as the quake brought huge disruption to Japan's key industries. Tokyo share prices plummeted and the yen was down against the dollar.
The first quake struck just under 400 kilometres (250 miles) northeast of Tokyo, the US Geological Survey said. It was followed by more than 60 aftershocks, one as strong as 7.1.
"We were shaken so strongly for a while that we needed to hold on to something in order not to fall," said an official at the local government of the hardest-hit city of Kurihara in Miyagi prefecture.
"We couldn't escape the building immediately because the tremors continued... City officials are now outside, collecting information on damage."
US President Barack Obama led international offers of sympathy and aid in what he called Japan's "time of great trial", while the Kan government called on help from US forces stationed in the country.
Japan sits on the "Pacific Ring of Fire", which is dotted with volcanoes, and Tokyo is in one of its most dangerous areas, where three continental plates are slowly grinding against each other, building up enormous seismic pressure.
The government has warned of a 70 percent chance that a great, magnitude-eight quake will strike within the next 30 years in the Kanto plains, home to Tokyo's vast urban sprawl.
The last time a "Big One" hit Tokyo was in 1923, when the Great Kanto Earthquake claimed more than 140,000 lives, many of them in fires. In 1855, the Ansei Edo quake also devastated the city.
In 1995 the Kobe earthquake killed more than 6,400 people.
More than 220,000 people were killed when a 9.1-magnitude quake hit off Indonesia in 2004, unleashing a massive tsunami that devastated coastlines in countries around the Indian Ocean.
However, small quakes are felt every day somewhere in Japan and people take part in regular drills at schools and workplaces to prepare for a calamity.