"I would like to ask the nation, although this incident is of great concern, I ask you to react very calmly," Kan said during a brief televised address.
Radiation levels around the plant Tuesday were measured at 8,217 microsieverts an hour -- more than 7,000 above the legal limit.
Anyone less than 20 kilometres of the reactors was urged to leave the area, while anyone within 20 to 30 km was told to stay inside, close all windows and turn off any air conditioners.
Despite the radiation levels, 50 workers remain at the power plant, locked in a struggle to cool the damaged reactors and prevent any catastrophic meltdowns.
