Third explosion deals another blow to Fukushima atomic plant after earthquake and tsunami crippled its cooling systems.
Explosions and fire have rocked quake-stricken Fukushima nuclear plant in northeast Japan, pumping out dangerous radiation and sparking panic in nearby town and cities.
Fearful citizens stripped supermarket shelves on Tuesday, prompting the government to warn against panic-buying, saying this could hurt the provision of relief supplies to quake-hit areas.
But scared Tokyo residents filled outbound trains and rushed to shops to stock up on food, water, face masks and emergency supplies amid heightening fears of radiation.
Radiation levels around the Fukushima No.1 plant on the eastern coast had "risen considerably", Naoto Kan, the prime minister said, and his chief spokesman announced the level was now high enough to endanger human health.
In Tokyo, some 250km to the southwest, authorities also said that higher than normal radiation levels had been detected in the capital, the world's biggest urban area, but not at harmful levels.
Kan warned people living up to 10km beyond a 20km exclusion zone around the nuclear plant to stay indoors.
"I would like to ask the nation, although this incident is of great concern, I ask you to react very calmly," he said.
Al Jazeera’s Steve Chao, reporting from Yamagata, said a no-fly zone has been established in a 30 km radius over the Fukushima nuclear plant.
The fire, which was later extinguished, broke out in the plant's number-four reactor, he said, meaning that four out of six reactors at the facility were in trouble - and temperatures were reportedly rising in the last two.
Damage is 'massive'
Radiation levels later dropped at both the plant and in Tokyo, Yukio Edano, the chief government spokesman, said.
As well as the atomic emergency, Japan is struggling to cope with the enormity of the damage from Friday's record quake and the tsunami that raced across vast tracts of its northeast, destroying all before it.
The official death toll rose to 2,414, police said on Tuesday, while officials said at least 10,000 were likely to have perished.
But the only country in the world to have experienced a nuclear attack - two bombs dropped by the US during World War II killed some 200,000 people - Japanese citizens are gripped by fear of nuclear fallout.
"What we most fear is a radiation leak from the nuclear plant," Kaoru Hashimoto, 36, a housewife living in Fukushima city 80km northwest of the stricken plant, said.
Hashimoto said supermarkets were open but shelves were completely empty.
"Many children are sick in this cold weather but pharmacies are closed. Emergency relief goods have not reached evacuation centres in the city.
"Everyone is anxious and wants to get out of town. But there is no more petrol."
More than 200,000 people have already been evacuated from the exclusion zone around the crippled plant.
At one shelter, a young woman holding her baby told public broadcaster NHK: "I didn't want this baby to be exposed to radiation. I wanted to avoid that, no matter what."
However, even in evacuation centres filled with quake-shocked and tsunami survivors, Japan's famed emphasis on social harmony is in evidence.
From the sharing of tasks among volunteers to the neat arrangement of shoes outside the living areas, life in the shelters is orderly and peaceful.
But the crisis at the ageing Fukushima nuclear plant has worsened daily since Friday's quake and tsunami knocked out cooling systems.
On Saturday an explosion blew apart the building surrounding the plant's NO.1 reactor.
On Monday, a blast hit the No.3 reactor, injuring 11 people and sending plumes of smoke billowing into the sky.
Early on Tuesday a third blast rocked the No.2 reactor. That was followed by a hydrogen explosion that started a fire at the No.4 reactor.
Expert assistance
Edano, the chief government spokesman, said radioactive particulates leaked along with the hydrogen.
The UN's nuclear watchdog, the International Atomic Energy Agency, said Tokyo had asked for expert assistance in the aftermath of the quake, which US seismologists are now measuring at 9.0-magnitude, revised up from 8.9.
Search intensifies for Japan survivors [Al Jazeera]
Aid workers and search teams from across the world have joined 100,000 Japanese soldiers in a massive relief push in the shattered areas.
Rising death toll
In the region of Miyagi, which took the full brunt of Friday's terrifying wall of water, rescue teams searching through the shattered debris of towns and villages have found 2,000 bodies.
And the Miyagi police chief has said he is certain more than 10,000 people perished in his prefecture alone.
Millions have been left without water, electricity, fuel or enough food and hundreds of thousands more are homeless and facing harsh conditions with sub-zero temperatures overnight, and snow and rain are forecast.