Panic swept Tokyo early today after a rise in radiation levels from an earthquake-hit nuclear power plant north of the city.
Some residents left the capital, and others were stocking up on food and supplies.
Several embassies advised staff and citizens to leave affected areas, tourists cut short vacations and multinational companies urged staff to leave or said they were considering plans to move outside Tokyo.
Radios, torches, candles, fuel cans and sleeping bags were sold out in the Don Quixote, a multistorey, 24-hour general store in downtown Tokyo.
Radiations 410 times legal limit
Overnight Japan's Chief Cabinet Secretary Yukio Edano said radiation levels were detected as high as 8,217 microsieverts per hour around the plant's main entrance yesterday - around 410 times the legal limit, the Japanese NHK World TV network reported.
Edano said that although the figure was serious, the level had been falling from its momentary peak.
Temperatures in the plant's 5 and 6 units were gradually increasing due to malfunctioning of cooling systems affected by the quake. Workers were trying to prevent possible hydrogen gas explosions similar to that at the number 4 reactor, NHK reported.
Water was being injected steadily at reactors 1 and 3 at the Fukushima nuclear plant, but the injection of water into the number 3 unit was not yet stable.
Tourists had enough
But tourists such as Christy Niver, of the United States, said they had enough and were leaving.
Her 10-year-old daughter, Lucy, was more emphatic. "I'm scared. I'm so scared I would rather be in the eye of a tornado," she said. "I want to leave."
New Zealand's Ministry of Foreign Affairs and Trade is now recommending against all tourist and other non-essential travel to areas in Japan, including Tokyo, affected by the earthquake and tsunami. China said it would evacuate expat citizens from the worst-hit areas of Japan.
Radioactive contamination fears
Kyodo News said "minute levels" of radiation had been detected in Tokyo, and radiation levels in Saitama, near Tokyo, were 40 times normal levels - not enough to affect human health, but enough to fuel panic in the city of about 12 million people.
Fears of radioactive contamination from the crippled nuclear plant in tsunami-ravaged Japan escalated after a third reactor explosion and a fire in another.
The International Atomic Energy Agency said last night Japanese officials had told it the reactor fire was in the storage pond - a pool where used nuclear fuel is kept cool - and that "radioactivity is being released directly into the atmosphere".
In the worst case, the reactor's core would completely melt down, a disaster that could spew large amounts of radioactivity into the atmosphere.
Prime Minister Naoto Kan said dangerous levels of radiation had spread from four reactors of the Fukushima nuclear plant, one of the hardest-hit in Friday's 9.0-magnitude earthquake and tsunami that has killed at least 20,000 people.
Residents told to stay indoors
Mr Kan warned of the dangers of more leaks and told people living within 30km of the complex to stay indoors to avoid radiation sickness.
About 70,000 people have been evacuated from a 20km radius and 140,000 remain in the zone for which the new warning was issued.
"The level seems very high and there is still a very high risk of more radiation coming out," Mr Kan said. "We are making utmost efforts to prevent further explosions and radiation leaks."
It is the worst nuclear crisis Japan has faced since the atomic bombing of Hiroshima and Nagasaki during World War II. And it is the first time such a grave nuclear threat has been raised since a nuclear power plant in Chernobyl, Ukraine, exploded in 1986.
'We are talking about levels that can damage human health'
Winds contaminated with low-level radiation from the reactors were last night drifting towards Tokyo, although not yet at harmful levels.
Far outside the disaster zone, stores are running out of necessities, raising Government fears that hoarding may hurt the delivery of emergency food aid to those who really need it.
Three of the six reactors at the Fukushima power plant were in critical condition after Friday's quake.
In a bid to stop the reactors melting down, engineers have been injecting seawater as a coolant of last resort.
But a fourth reactor caught fire for a period yesterday and more radiation was released.
"It is likely that the level of radiation increased sharply due to a fire at Unit 4," said Chief Cabinet Secretary Yukio Edano.
"Now we are talking about levels that can damage human health. These are readings taken near the area where we believe the releases are happening. Far away, the levels should be lower.
"These are figures that potentially affect health. There is no mistake about that.
"There were no fuel rods in the reactor, but spent fuel rods are inside."
Spent fuel rods can still emit large amounts of radioactive material and need to remain immersed in cool water.
But although the No 4 reactor was emitting large amounts of radioactivity, Mr Edano said it "did not pose an imminent threat".
The sharp deterioration came after a frantic day and night of rescue efforts focused largely on the No 2 reactor.
There, a malfunctioning valve prevented workers from manually venting the containment vessel to release pressure and allow fresh seawater to be injected into it.
That meant the remedy emergency workers had been using to keep the nuclear fuel from overheating no longer worked.
As a result, the nuclear fuel in that reactor was exposed for many hours, increasing the risk of a breach of the container vessel and more dangerous emissions of radioactive particles.
Officials said 50 workers were trying to put water into the reactors to cool them. The fires and explosions at the reactors have injured 15 workers and military personnel and exposed up to 190 people to elevated radiation.
The crisis is rated 4 on a nuclear safety rating on a scale of 1 to 7 - less than Three Mile Island, a 5, and Chernobyl at 7.
At Three Mile Island, the radiation leak was held inside the containment shell - thick concrete armour around the reactor. The Chernobyl reactor had no shell and was also operational when the disaster struck. The Japanese reactors automatically shut down when the quake hit and are encased in containment shells.
The death toll from last week's earthquake and tsunami topped 2400, though that grim news was overshadowed by the deepening nuclear crisis.
Officials have said previously that at least 10,000 people may have died in Miyagi province alone.
- Agencies, staff reporter