Japan ordered nuclear reactors back online on Saturday, defying public sentiment against atomic power following last year's meltdowns at Fukushima sparked by a huge quake-tsunami disaster.

The third and fourth reactor building of the Ohi nuclear power plant

Prime Minister Yoshihiko Noda on Saturday received approval for the restarts from Issei Nishikawa, the pro-nuclear power governor of central Fukui prefecture, which hosts the plant.

The premier then met three ministers -- the minister of economy, trade and industry, the minister in charge of the nuclear accident and the chief cabinet secretary.

"Now that we have the approval from the autonomous body where the reactors are relocated, the four ministers (including Noda) concerned made the decision to restart the reactors," Noda told the meeting on camera.

Nishikawa told the prime minister he was happy with the restarts after he received safety assurances on Friday from the operator.

"We reached the agreement to help stabilise livelihoods and industry in Kansai (western Japan)," Nishikawa said.

The controversial move comes amid fears that electricity demand will outstrip supply as temperatures soar and air-conditioners get cranked up, further crimping Japan's wobbly economic recovery.

About 500 hundred people rallied outside Noda's official residence in central Tokyo to protest against his approach to nuclear power generation despite the on-going nuclear accident.

"Don't activate dangerous nuclear reactors any more," their banners read.

But Noda, seeking to head off a summer energy crunch, told Kansai Electric Power (KEPCO) to re-fire two idled reactors at its Oi plant serving the industrial heartland of western Japan.

According to an opinion poll conducted by public broadcaster NHK and released on Monday, 25 percent of 1,079 respondents supported the restart of the Oi reactors while 32 percent opposed it and 38 percent had no opinion.

The nod from Nishikawa was the final link in the chain for Noda, who has become a vocal advocate of nuclear power being brought back into the energy mix for resource-poor but electricity-hungry Japan.

The country's 50 working reactors -- which along with the four crippled units at Fukushima contributed around a third of Japan's electricity before the disaster -- have been offline since the last one was shuttered in early May.

Public opposition in the aftermath of the tsunami-sparked meltdowns at Fukushima in March 2011 left Japan's political classes tip-toeing around the issue of restarts.

Radiation was spread over homes and farmland in a large swathe of northern Japan when the massive tsunami swamped cooling systems at Fukushima Daiichi.

No one is officially recorded as having died as a direct result of the meltdowns, but tens of thousands of people were evacuated and many remain so, with warnings some areas will be uninhabitable for decades.

Anti-nuclear sentiment among the public has run into increasingly apocalyptic warnings of power shortfalls, the most dire of which predicted Kansai's manufacturing base could see a one-fifth gap.

KEPCO has cautioned this will mean blackouts, which are expected to wallop producers already struggling against a tide of economic uncertainty and export markets stumbling under the pressure of Europe's debt crisis.

However, Noda's conviction that Japan could not do without nuclear power was not enough, forcing him to seek cover from international bodies and local politicians.

On Friday, Japan's Nobel literature prize laureate Kenzaburo Oe visited the prime minister's office and handed the signatures of 6.5 million opposed to the continued use of nuclear reactors.