Prime Minister Naoto Kan has said Japan is experiencing its greatest hardships since World War II as it tackles the aftermath of an earthquake, tsunami and a growing nuclear crisis.
He said the situation at the quake-hit Fukushima nuclear plant remained grave, a day after an explosion at a reactor.
Japanese broadcaster NHK says the total number of confirmed deaths caused by the disaster now stands at 1,596.
But police warn that the death toll in Miyagi region alone could top 10,000.
The authorities are stepping up relief efforts as the scale of the tragedy becomes clearer.
Huge numbers of survivors are gathered in emergency shelters, and many people are without fresh running water, heat and power.
Risk of explosion
The cooling systems of two reactors at the Fukushima No.1 nuclear power plant failed after the 8.9-magnitude quake struck off the north-east coast. Both are in danger of melting down with potentially serious environmental consequences.
Sea water is being injected into reactors 1 and 3 in an attempt to cool them - a last-ditch move that will render the reactors unusable.
On Saturday, a hydrogen explosion blew apart the building housing reactor 1, where technicians had been releasing radioactive steam as part of their attempts to cool the reactor.
Government officials have admitted there is now a risk of a similar explosion in the building housing reactor 3. However, chief cabinet secretary Yukio Edano said the facility could withstand the impact and the nuclear reactor itself would not be damaged.
The authorities say that radiation levels around the damaged plant have exceeded legal safety limits.
Tens of thousands of people are being evacuated from within a 20km (12.4-mile) radius.
At least 22 people are now said to be being treated for the effects of exposure to radiation. Workers in protective clothing have been checking residents as they leave the evacuation zone.
Earlier, Japan's nuclear energy agency declared a state of emergency at a second nuclear facility, at Onagawa, after excessive radiation levels were recorded there. However, the agency now says levels have returned to normal.
It said cooling systems at all three reactors at the Onagawa complex, which were automatically shut down after the earthquake and tsunami, were functioning properly and the rise in local radiation levels might have been caused by the Fukushima leak.
'Most severe crisis'
"The current situation of the earthquake, tsunami and the nuclear plants is in a way the most severe crisis in the past 65 years since World War II," Mr Kan said.
"Whether we Japanese can overcome this crisis depends on each of us.
"I strongly believe that we can get over this great earthquake and tsunami by joining together."
Mr Kan said the shutting down of Fukushima No.1 and other nuclear power stations would entail rolling power cuts nationwide, and urged citizens to conserve energy.
He said that from Monday there would be a programme of rolling power cuts that would also affect water and gas supplies and some medical facilities.
The number of troops helping with rescue work in the affected north-east coastal region is being doubled to 100,000. They will be joined by 250,000 police officers and other relief workers.
International rescue teams are flying into Japan following an appeal by the government, while US forces based in thre country are helping to transport relief supplies to survivors.
Rescue workers have found scenes of total devastation in isolated coastal towns north-east of the main port city of Sendai, which was itself partially destroyed by the waves.
The BBC's Rachel Harvey, who is trying to travel along the main coast road, says huge piles of debris and wreckage are blocking the route.
But some survivors are being found. Japanese troops rescued a 60-year-old man who floated out to sea on the roof of his home after the tsunami hit.
Hiromitsu Shinkawa was pulled from the sea about 15km off the town of Minamisoma, in Fukushima prefecture, after he was spotted waving a red cloth.
Mr Shinkawa told his rescuers that the tsunami had hit as he and his wife returned home to gather some posessions after the earthquake, and that his wife was swept away.
He was reported to be in good health despite his two-day ordeal.
Preliminary estimates put repair costs from the earthquake and tsunami in the tens of billions of dollars, a huge blow for the Japanese economy that - while still the world's third largest - has been ailing for two decades.