Prime Minister Naoto Kan said on Saturday that residents living within 10 kilometres of the plant must evacuate amid fears of a slight radiation leak, before stepping onto a helicopter to head for the area.
An explosion was heard on Saturday from a quake-hit Japanese nuclear plant and smoke was seen billowing from it, as several workers were injured and radioactivity rose 20-fold outside, AFP said.
The atomic emergency came as the country struggled to assess the full extent of the devastation wrought by the massive tsunami triggered by the strongest quake ever recorded in Japan.
Tsunami trail of devastation
The towering wall of water generated by the 8.9-magnitude earthquake - the seventh biggest in history - pulverised the northeastern city of Sendai, where police on Friday reportedly said that 200-300 bodies had been found on the coast. Kyodo News said the final death toll was likely to pass 1000.
The 10-metre 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 202 people had been confirmed dead and 673 missing, with 991 injured in the massive quake and tsunami disaster that devastated large parts of northern Japan's Pacific coast.
"The damage is so enormous that it will take us much time to gather data," an official at the agency told AFP.
The tsunami left Rikuzen Takata, a coastal city of some 23,000 people, "almost in shambles," the national Fire and Disaster Management Agency said.
Video footage, taken by a resident from a hill, showed massive waves sweeping away houses, vehicles and other objects as they surged over the city from the sea.
Before the prime minister told residents around the Fukushima nuclear plant to evacuate, trade minister Banri Kaieda warned there could be a release of radioactive steam in an operation to relieve a pressure build-up after the plant's cooling system was damaged in the quake.
Japan's network of advanced nuclear power plants are designed to shut down as soon as the earth shakes in one of the world's most quake-prone countries, though a fire broke out in the turbine building of another nuclear plant in Onagawa.
More than eight million homes lost power, mobile and landline phone systems broke down for many and gas was cut to more than 300,000 homes, meaning many people could not heat their dark homes during a terrifying and cold night.
Huge relief mission launched
Japan's military mobilised thousands of troops, 300 planes and 40 ships for the relief effort.
An armada of 20 naval destroyers and other vessels headed for the devastated Pacific coast area of Honshu island, while air force jets flew reconnaissance missions.
Leading international offers of help, President Barack Obama mobilised US military might to provide emergency aid after the disaster which he described as "simply heartbreaking."
The United States, which has nearly 40,000 military personnel in Japan, has ordered a flotilla including two aircraft carriers and support ships to the region to provide aid following the tsunami.
International search and rescue teams rushed to Japan Saturday in the wake of a massive 8.9-magnitude earthquake and crushing tsunami, some straight from similar work in quake-hit New Zealand.
A Japanese team of 66 personnel which has spent more than two weeks scouring the rubble left by last month's 6.3-magnitude quake in Christchurch was making hasty preparations to return home to confront the unfolding tragedy.
The United States said it was sending nearly 150 rescue workers to the disaster zone, among them a team from Los Angeles that had only returned from New Zealand two days ago.
Australia, South Korea and Singapore will all send dogs and search and rescue teams, their governments said Saturday as they expressed their condolences to Tokyo.
Tsunamis roll across the Pacific
The towering wave set off alerts across the Pacific, sparking evacuations in Hawaii and the US West Coast, and devastating at least one California port.
Chile said it was evacuating coastal areas and Ecuador's state oil company announced it had suspended crude oil exports due to risks posed by the tsunami.
The largest earthquake ever recorded in Japan
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 quake, which hit at 2.46pm (1646 AEDT) and lasted about two minutes, rattled buildings in greater Tokyo, the world's largest urban area and home to some 30 million people. It was felt in Beijing, some 2500km away.
Millions who had earlier fled swaying buildings in the capital were left stranded in the evening after the earthquake shut down the city's vast subway system.
The government urged 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 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 tsunami also submerged the runway at Sendai airport, while a process known as liquefaction, caused by the intense shaking of the tremor, turned parts of the ground to liquid.
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 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 400km northeast of Tokyo, the US Geological Survey said. It was followed by more than 70 powerful 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."
Japan sits on the "Pacific Ring of Fire" 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 per cent chance that a 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 1995 the Kobe earthquake killed more than 6400 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.
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.
