The warning came as an explosion rocked the nearby Fukushima Dai-ichi nuclear power plant. The blast was felt 30 miles (50 kilometers) away by Associated Press journalists in the coastal town of Soma, where residents fled the town for safety after being herded quickly through muddy, debris-strewn streets.
TV footage showed a massive column of smoke belching from the Fukushima Dai-ichi nuclear plant's No. 3 unit, about 125 miles (190 kilometers) north of Tokyo. Japanese officials said they believe it was a hydrogen explosion similar to an earlier one at a different unit in the facility. The problems at the plant stem from failed cooling systems caused by damage from Friday's earthquake and tsunami.
More than 180,000 people have evacuated the area, and up to 160 may have been exposed to radiation.
Before the power plant blast, sirens around Soma, which was battered by Friday's tsunami, went off and public address systems ordered residents to safety.
Farther south along the coast, helicopters flew over coastal communities warning residents to head to higher ground. In Sendai, the biggest city in the area, police announced warnings on a public address system.
In Tokyo and elsewhere, authorities began rolling blackouts to conserve power as they tried desperately to stabilize the nuclear reactors at risk of meltdown in the aftermath of the earthquake and tsunami. The disasters sent Tokyo's stock market plunging as it opened, raising fears of a steep economic toll on top of the already overwhelming human suffering.
The planned blackouts of about three hours each in Tokyo and other cities are meant to help make up for the loss of power from key nuclear plants. Trade Minister Banri Kaieda said Sunday that the power utility expects a 25 percent shortfall.
Some 1.9 million households were without electricity, but many people were without even more basic necessities. At least 1.4 million households had gone without water since the quake struck, and food aid was slow in reaching many areas.
Friday's quake and tsunami, which swallowed towns and tossed large ships like game-board pieces, caused tens of billions of dollars in losses, according to preliminary estimates. And the first day of stock trading since the disasters opening underlined the challenges Japan's already fragile economy will have in bouncing back.
The benchmark Nikkei 225 stock average shed 494 points, or 4.8 percent, to 9,760.45 just after the market opened Monday. Japan's central bank quickly responded by injecting 7 trillion yen (US$85.5 billion) into money markets.
The most urgent crisis remained at a nuclear plant along the ravaged northeastern coast, where operators worked frantically to try to lower temperatures of crippled reactors. Four nuclear plants had at least some damage, but two reactors at the Fukushima Dai-ichi nuclear complex were at the greatest risk of meltdown.
Operators dumped seawater into the two reactors in a last-ditch cooling effort. More than 180,000 people have evacuated the area, and up to 160 may have been exposed to radiation.
Officials have confirmed about 1,800 deaths from the earthquake and tsunami - including 200 people whose bodies were found Sunday along the coast - and said more than 1,700 were missing and 1,900 injured.
The death toll seemed certain to get much higher after a report from Miyagi, one of the three hardest hit states. The police chief estimated that more than 10,000 people were killed there, police spokesman Go Sugawara told The Associated Press. Only about 400 people in the state of 2.3 million have been confirmed dead so far.
Todd Pitman reported from Sendai. Associated Press writers Eric Talmadge and Kelly Olsen in Koriyama and Malcolm J. Foster, Mari Yamaguchi, Tomoko A. Hosaka and Shino Yuasa in Tokyo contributed to this report.
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