About 20,000 people gathered in front of Japanese Prime Minister Yoshihiko Noda's residence in Tokyo late Friday to protest his decision to restart two nuclear reactors.

"No to the restart!" shouted the protestors, who were led by investigative journalist Satoshi Kamata and Nobel Prize-winning author Kenzaburo Oe, who started an anti-nuclear petition that has so far gathered more than 7.5 million signatures.

Last Saturday, Noda gave the green light to start work to put back online two reactors at the Oi plant in western Japan, despite public distrust in the technology since last year's meltdowns at Fukushima.

Japan had been left without nuclear power since early May when the last of its 50 working reactors was shut down. Authorities took the decision to restart the two reactors as they seek to head off a summer power crunch.

Friday's protest was the latest sign of unease over the decision which was taken in conjunction with local authorities and despite the fact that Noda had previously vowed not to act without public backing.

Protesters said they would hold another demonstration next week.

"The battle has only just begun", insisted renowned composer Ryuichi Sakamoto, another leading figure in the anti-nuclear movement.

Radiation was spread over homes and farmland in a large swathe of northern Japan when a 9.0-magnitude earthquake and ensuing tsunami in March last year 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.