The developments raised fears of a meltdown at the plant as officials scrambled to contain what could be the worst nuclear disaster since the Chernobyl explosion in 1986 that shocked the world.
The Japanese plant was damaged by Friday's 8.9-magnitude earthquake, which sent a 10-meter (33-foot) tsunami ripping through towns and cities across the northeast coast. Japanese media estimate that at least 1,300 people were killed.
"We are looking into the cause and the situation and we'll make that public when we have further information," Chief Cabinet Secretary Yukio Edano said after confirming the explosion and radiation leak at the plant.
Edano said an evacuation radius of 10 km (6 miles) from the stricken 40-year-old Daiichi 1 reactor plant in Fukushima prefecture was adequate, but an hour later the boundary was extended to 20 km (13 miles). TV footage showed vapor rising from the plant, 240 km (150 miles) north of Tokyo.
The explosion at Chernobyl's nuclear plant's fourth reactor in 1986 sent thousands of tons of toxic nuclear dust billowing across the Ukraine, Russia and Belarus. It was the worst civil nuclear disaster.
The blast at the Japanese nuclear facility came as plant operator Tokyo Electric Power Co (TEPCO) worked desperately to reduce pressures in the core of the reactor.
The company has had a rocky past in an industry plagued by scandal. In 2002, the president of the country's largest power utility was forced to resign along with four other senior executives, taking responsibility for suspected falsification of nuclear plant safety records.
NHK television and Jiji news agency said the outer structure of the reactor building that houses the reactor appeared to have blown off, but nuclear experts said this did not necessarily mean the nuclear reactor had been breached.