Panic selling sent Tokyo shares down 12.18% on worries Japan's nuclear crisis would become a catastrophe today, after radiation levels near a quake-stricken nuclear plant surged following explosions and a fire.
The Nikkei index lost 1,171.63 points to 8,448.86 by around 0345 GMT.
The Topix index of all first section shares lost 12.52 percent, or 106.07 points, to 740.89.
Prime Minister Naoto Kan said there was a high risk of elevated levels of radiation from a reactor at the Fukushima nuclear power plant that exploded earlier in the day.
He urged people within 30 kilometres of the plant to stay indoors.
Authorities said an explosion at the No. 2 reactor at the Fukushima Daiichi nuclear power plant, located 160 miles northeast of Tokyo, appears to have caused damage.
Some staff had been evacuated from the facility as radiation levels at the site rose sharply.
In volatile trade the yen dived against the dollar before snapping back as traders worried about the escalating nuclear emergency.
The dollar rose to 82.01 before falling back to 81.47.
