Japan fought on Sunday to avert a disastrous meltdown at two earthquake-crippled nuclear reactors as estimates of the death toll from the tsunami that charged across its northeast rose to more than 10,000.

Broadcaster NHK, quoting a police official, said more than 10,000 people may have been killed as the wall of water triggered by Friday's 8.9-magnitude quake surged across the coastline, reducing whole towns to rubble.

Officials worked desperately to prevent the fuel rods in the  damaged plants from overheating after radiation leaked into the air. The government said a building housing a second reactor was at risk of exploding after a blast blew the roof off a different plant the day before.

The nuclear accident, the worst since the Chernobyl disaster in 1986, sparked stinging criticism that authorities were ill-prepared for such a massive quake and the threat that could pose to the country's nuclear power industry. 

Thousands were evacuated on Saturday following the explosion and leak from the facility's No. 1 reactor in Fukushima, 240 km (150 miles) north of Tokyo, where there is believed to have been a partial meltdown of the fuel rods.

Engineers were pumping in seawater, trying to prevent the same happening at the No. 3 reactor, the government said in apparent acknowledgement it had moved too slowly on Saturday.

'Unlike the No.1 reactor, we ventilated and injected water at an early stage,' Chief Cabinet Secretary Yukio Edano told a news briefing.

Asked if fuel rods were partially melting in the No. 1  reactor, Edano said: 'There is that possibility. We cannot confirm this because it is in the reactor. But we are dealing with it under that assumption .'

He said fuel rods may have partially deformed at the No. 3 reactor but a meltdown was unlikely to have occurred.

'The use of seawater means they have run out of options,' said David Lochbaum, director of the Union of Concerned Scientists Nuclear Safety Project.      'If they had any other water they would have used it. It likely means the power for their pumps is gone.' 

Nuclear plant operator Tokyo Electric Power (Tepco) said radiation levels around the Fukushima Daiichi plant had risen above the safety limit but that it did not mean an 'immediate threat' to human health.

Edano said there was a risk of an explosion at the building housing the No. 3 reactor, but that it was unlikely to affect the reactor core container.     

The government said it planned electricity blackouts in areas covered by Tepco lasting a few weeks.

The disaster prompted an angry response from an anti-nuclear energy NGO in Japan which said it should have been foreseen. 'A nuclear disaster which the promoters of nuclear power in Japan said wouldn't happen is in progress,' the Citizens' Nuclear Information Centre said. 'It is occurring as a result of an earthquake that they said would not happen.'     

Thousands spent another freezing night huddled in blankets over heaters in emergency shelters along the northeastern coast, a scene of devastation after the quake sent a 10-metre (33-foot) wave surging through towns and cities in the Miyagi region, including its main coastal city of Sendai.     

In one of the heavily hit areas, Rikuzentakata, a city close to the coast, more than 1,000 people took refuge in a school high on a hill. Some were talking with friends and family around a stove. The radio was giving updates. On the walls were posters where names of survivors at the shelter were listed.      Some were standing in front of the lists, weeping.     

Kyodo news agency reported there had been no contact with around 10,000 people in one town, more than half its population.     

A Japanese official said there were 190 people within a 10-km radius of the nuclear plant when radiation levels rose and 22 people have been confirmed to have suffered contamination.
 Workers in protective clothing were scanning people arriving at evacuation centres for radioactive exposure.

The crisis has triggered anti-nuclear power protests in Europe. Up to 60,000 protesters formed a 45-km (27-mile) human chain in Germany to denounce the government's policy of extending the life of nuclear plants.       

Officials in Japan ordered the evacuation of a 20-km (12-mile) radius zone around the nuclear plant and 10 km (6 miles) around another nuclear facility close by.      

Around 140,000 people had left the area, while authorities prepared to distribute iodine to protect people from radioactive exposure.

'There is radiation leaking out, and since the possibility (of being exposed) is high, it's quite scary,' said Masanori Ono, 17, standing in line on Saturday to be scanned for radiation at an evacuation centre in Fukushima prefecture.     

The wind over the plant would continue blowing from the south, which could affect residents north of the facility, an official at Japan's Meteorological Agency said.

The government, in power less than two years and which had already been struggling to push policy through a deeply divided parliament, came under criticism for its handling of the crisis.

'Crisis management is incoherent,' blared a headline in the Asahi newspaper, charging that information disclosure and instructions to expand the evacuation area around the troubled plant were too slow.

'Every time they repeated 'stay calm' without giving concrete data, anxiety increased,' it quoted an unidentified veteran party lawmaker as saying. - Reuters