The government announced the power supply warning on Monday night. It's the first alert of its kind to be issued in Japan.

Economy, Trade and Industry Minister Hagiuda Koichi asked for public cooperation to reduce power use. He told reporters on Tuesday morning that supply was expected to be extremely tight.

Households and companies were asked to save power between 8 a.m. and 11 p.m. on Tuesday in Tokyo and eight other prefectures, where rain or snow are forecast and temperatures are expected to dip.

Aside from Tokyo, the affected prefectures are Ibaraki, Tochigi, Gunma, Saitama, Chiba, Kanagawa, Yamanashi and parts of Shizuoka Prefecture east of Fujikawa river.

The benchmark for a stable power supply is that usage is kept below 97 percent of the supply capacity. The government warned that a mass power outage could occur if supply capacity cannot be secured.

It said a 10 percent reduction of power usage would ensure a stable supply. It asked residents and businesses to switch off unnecessary lights and heaters, set thermostats at 20 degrees Celsius, and dim the brightness of TVs.

All the affected areas lie within the service area of Tokyo Electric Power Company. Two of its thermal power units in Fukushima Prefecture have been suspended since the quake last Wednesday.

TEPCO said it will raise output to maximum levels at other thermal plants to counter shortages. But it has asked users to help save power, as the utility needs more time to put suspended power units back online.