Dangerous levels of radiation leaking from a crippled nuclear plant have forced Japan to order 140,000 people to seal themselves indoors 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, Tuesday'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.
Conditions at the crippled nuclear power plant have 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
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 170 miles (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 on Tuesday morning, according to Kyodo News agency. 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 19 miles (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."