Officials warned that there was a "high possibility" of meltdown at the ageing facility north of Tokyo, which was rocked by an explosion yesterday following an 8.9-strength tremor that sent 10-metre waves bulldozing inland.

Backup cooling systems failed, leaving the core to glow unchecked and sparking fears that fuel could breach the containment shell, leaking dangerous radiation into the densely-populated region that houses 30 million people.

About 200,000 residents were evacuated from a 20-kilometre radius around the plant, which was built in the 1970s and is one of 54 nuclear plants providing about 30 per cent of Japan's power.

Operator Tokyo Electric Power Co (TEPCO) insisted radiation was still within safe levels, but mounting internal pressure meant that some vapour had to be released, and it warned another blast might take place in a second reactor.

Anti-nuclear campaigners said the crisis was a timely reminder of the dangers of atomic energy, particularly in a seismic hotspot like Japan, with Greenpeace describing it as an "inherently hazardous" industry.

Tens of thousands of demonstrators took to the streets in Germany over plans to prolong the country's dependency on nuclear power, while Russia ordered a review of its emergency response procedures.

Beijing said it was watching developments closely, having stepped up investment in nuclear power in a bid to slash carbon emissions, with 27 plants being built, 50 in the planning phase and another 110 proposed.

According to the World Nuclear Association there are 443 nuclear reactors operating worldwide, with another 62 under construction, 158 on order and 324 proposed.

Peter Bradford, a former member of the US Nuclear Regulatory Commission, said the disaster was "obviously a significant setback for the so-called nuclear renaissance".