An explosion blew the roof off an unstable reactor north of Tokyo, raising fears of a meltdown at a nuclear plant damaged in the massive earthquake that hit Japan.
The 8.9-magnitude earthquake - the strongest ever recorded in Japan - sent a 10-metre tsunami ripping through towns and cities across the northeast coast.
Japanese media estimate that at least 1300 people were killed.
Jiji news agency said there had been an explosion at the stricken 40-year-old Daichi 1 reactor and television footage showed vapour rising from the plant, which lies 240 km north of Tokyo.
The Tokyo Electric Power Company says that several workers were injured reports Japan's public broadcaster NHK.
The blast came as plant operator Tokyo Electric Power Co (Tepco) worked desperately to reduce pressures in the core of the reactor that - if not contained - could lead to a release of radiation into the atmosphere.
NHK television and Jiji reported that the outer structure of the building that houses the reactor appeared to have blown off, which could suggest the containment building surrounding the reactor had already been breached.
Earlier the operator released what it said was a tiny amount of radioactive steam to reduce the pressure and the danger was minimal because tens of thousands of people had already been evacuated from the vicinity.
Japan's Nuclear and Industrial Safety Agency said two radioactive substances, cesium and radioactive iodine had been detected at one of the reactors at the Fukushima 1 plant reports NHK.
The evacuation zone around the plants was extended from 3 kilometres to 10.
Analysts say there is a low risk of a wide-scale nuclear disaster because the reactors are light-water units, meaning an explosion is unlikely.
Radiation levels inside one of the reactors were recorded at 1000 times their normal levels, and outside the plant at eight times their normal amount.