Japan started work to put nuclear reactors back online Saturday, despite public distrust of the technology after last year's meltdowns at Fukushima, the world's worst atomic accident since Chernobyl.

Prime Minister Yoshihiko Noda, seeking to head off a summer energy crunch, told Kansai Electric Power (KEPCO) to put into operation two idled reactors at its Oi plant, which serves the industrial heartland of western Japan.

The utility, which supplies Osaka and the surrounding areas, began preparatory work at 2:30 pm (0530 GMT) to reactivate one of the reactors, a KEPCO official said. Work on the other reactor will begin on June 21.

The first reactor is expected to start generating power as soon as July 4, he said.

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

But it has angered many Japanese wary of nuclear energy after the crisis sparked by the huge quake-tsunami disaster in March last year.

About 500 people rallied outside Noda's official residence in central Tokyo, carrying placards reading: "Don't activate dangerous nuclear reactors".

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

Noda received approval earlier in the day for the restarts from Issei Nishikawa, the pro-nuclear power governor of 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 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 all been offline since the last one was shut down in early May.

Public opposition 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 a 9.0-magnitude earthquake and ensuing tsunami crippled the cooling system of the Fukushima Daiichi plant.

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.

But anti-nuclear sentiment among the public has run into increasingly dire warnings of power shortfalls, the worst of which predicted Kansai's manufacturing base could see a one-fifth gap between demand and supply.

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

But in spite of his conviction that Japan could not do without nuclear power Noda has still felt it necessary to seek political cover from international bodies and local politicians.

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

The Japan branch of environment watchdog Greenpeace, meanwhile, urged Noda to "climb out of the pocket of the nuclear industry and put all of his energy into rapidly moving Japan towards the renewable energy future".