Dangerous levels of radiation leaking from a crippled nuclear plant forced Japan to order 140,000 people to seal themselves indoors today after an explosion and fire dramatically escalated the crisis spawned by the earthquake and tsunami four days ago.
In a nationally televised statement, Prime Minister Naoto Kan said radiation had spread from four reactors at the Fukushima Dai-ichi nuclear plant in Fukushima state, one of the hardest-hit by Friday's 9.0-magnitude earthquake and the ensuing tsunami which has killed more than 10,000 people, plunged millions into misery and pummelled the world's third-largest economy.
Although Mr Kan and other officials urged calm, today's developments fuelled a growing panic in Japan and around the world amid widespread uncertainty over what would happen next.
In the worst-case scenario, the reactor's core would completely melt down, a disaster which would spew large amounts of radioactivity into the atmosphere.
Tokyo reported slightly elevated radiation levels, but officials said the increase was too small to threaten the 39 million people in and around the capital, about 270km away.
Closer to the stricken nuclear complex, the streets in the coastal city of Soma were empty as the few residents who remained there heeded the government's warning to stay indoors.
Officials just south of Fukushima reported up to 100 times the normal levels of radiation this morning, Kyodo News agency reported. While those figures are worrying if there is prolonged exposure, they are far from fatal.
Mr Kan and other officials warned there is a danger of further leaks and told people living within 30km of the Fukushima Dai-ichi complex to stay indoors to avoid exposure that could make people sick.
"Please do not go outside. Please stay indoors. Please close windows and make your homes airtight," Chief Cabinet Secretary Yukio Edano told residents in the danger zone.
"These are figures that potentially affect health. There is no mistake about that," he said.
Weather forecasts for Fukushima were for snow and wind from the north-east this evening, blowing south-west towards Tokyo, and then shifting and blowing west out to sea. That is important because it shows which direction a possible nuclear cloud might blow.
The nuclear crisis is the worst Japan has faced since the atomic bombing of Hiroshima and Nagasaki during the Second World War. It is also the first time that such a grave nuclear threat has been raised in the world since a nuclear power plant in Chernobyl, Ukraine exploded in 1986.
Some 70,000 people had already been evacuated from a 20km radius around the Dai-ichi complex. About 140,000 remain in the new warning zone.
Workers were desperately trying to stabilise three reactors at the power plant which exploded in the wake of Friday's quake and tsunami, after losing their ability to cool down and releasing some radiation. Since the quake, engineers have been injecting seawater into the reactors as a last-ditch coolant.
A fourth reactor which had been shut down before the quake caught fire today and more radiation was released, Mr Edano said. The fire was later put out.
Even though the fourth reactor was shut down, the fire there was believed to be the source of the elevated radiation.
"It is likely that the level of radiation increased sharply due to a fire at Unit 4," Mr Edano said. "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."
He said another reactor whose containment building exploded yesterday had not contributed greatly to the increased radiation.
Officials said 50 workers, all of them wearing protective radiation gear, were still trying to pump water into the reactors to cool them. They said 800 other staff were evacuated.
The fires and explosions at the reactors have injured 15 workers and military personnel and exposed up to 190 people to elevated radiation.
In Tokyo, slightly higher-than-normal radiation levels were detected today but officials insisted there are no health dangers.
"The amount is extremely small, and it does not raise health concerns. It will not affect us," Takayuki Fujiki, a Tokyo government official said.
Kyodo reported that radiation levels nine times higher than normal were briefly detected in Kanagawa prefecture near Tokyo and that the Tokyo metropolitan government said it had detected a small amount of radioactive materials in the air.
The death toll from last week's earthquake and tsunami jumped today as police confirmed the number killed had topped 2,400, although that grim news was overshadowed by a deepening nuclear crisis. Officials have said previously that at least 10,000 people may have died in Miyagi province alone.
Millions of people spent a fourth night with little food, water or heating in near-freezing temperatures as they dealt with the loss of homes and loved ones. Asia's richest country has not seen such hardship since the Second World War.
Hajime Sato, a government official in Iwate prefecture, one of the hardest-hit, said deliveries of supplies were only 10% of what is needed. Body bags and coffins were running so short that the government may turn to foreign funeral homes for help, he said.
Though Japanese officials have refused to speculate on the death toll, Indonesian geologist Hery Harjono, who dealt with the 2004 Asian tsunami, said it would be "a miracle really if it turns out to be less than 10,000" dead.
The 2004 tsunami killed 230,000 people - but only 184,000 bodies were found.
The impact of the earthquake and tsunami dragged down stock markets. The benchmark Nikkei 225 stock average plunged for a second day today, nosediving more than 10% to close at 8,605.15 while the broader Topix lost more than 8%.
To reduce the damage, Japan's central bank made two cash injections totalling 8 trillion yen (70.8 billion euros) into the money markets today.
Initial estimates put repair costs in the tens of billions of dollars, costs that are likely to add to a massive public debt which , at 200% of gross domestic product, is the biggest among industrialised nations.
Yuta Tadano, a 20-year-old pump technician at the Fukushima plant, said he was in the complex when quake hit.
"It was terrible. The desks were thrown around and the tables too. The walls started to crumble around us and there was dust everywhere. The roof began to collapse.
"We got outside and confirmed everyone was safe. Then we got out of there. We had no time to be tested for radioactive exposure. I still haven't been tested," he told The Associated Press at an evacuation centre.
"I worry a lot about fall-out. If we could see it we could escape, but we can't," said Mr Tadano, cradling his four-month-old baby, Shoma.
The Dai-ichi plant is the most severely affected of three nuclear complexes which were declared emergencies after suffering damage in Friday's quake and tsunami, raising questions about the safety of such plants in coastal areas near fault lines and adding to global jitters over the industry.
Conditions at the crippled nuclear power plant deteriorated further with fears the water inside the waste fuel storage pool for one of the damaged reactors may be boiling.
A spokesman said "we cannot deny the possibility of water boiling" in the spent fuel storage pool.
If the water boils, it could evaporate, exposing the rods. The fuel rods are encased in safety containers meant to prevent them from resuming nuclear reactions, nuclear officials said, downplaying the risk of that happening.
But they acknowledged that there could have been damage to the containers. They also confirmed that the walls of the storage pool building were damaged.
The temperature of the water in the spent fuel storage pool for Unit 4 was 84C yesterday, when it was last measured. No measurements have been available since then, the spokesman said, adding: "We have no information about whether the spent fuel rods are exposed."