Japan prepared to bring nuclear power back online by reactivating one of its idled reactors on Sunday, defying growing public protests since last year's meltdowns at Fukushima.

Hundreds of protesters blocked the road to the front gate of the Oi plant in western Japan, according to media reports, with work scheduled to begin at 9:00 pm (1200 GMT) to remove rods that have stopped nuclear fission in the reactor.

The country has been operating without nuclear power since early May when the last of its 50 working reactors was shut down due to regular safety checks.

The nuclear restart had been on hold as the government mulled its options following a 9.0-magnitude earthquake and tsunami that crippled reactor cooling systems at Fukushima.

But on June 16 Prime Minister Yoshihito Noda gave the green light to restart two reactors at the Oi plant in western Japan in an effort to head off a summer power crunch.

The No. 3 reactor at Oi was expected to reach criticality around 6:00 am Monday (2100 GMT Sunday) and start supplying electricity on Wednesday, according to the plant's operator Kansai Electric Power Co. (KEPCO).

The 1.18 million-kilowatt reactor will be fully back online in about one week, KEPCO said.

The utility, which serves the industrial and commercial hub of Osaka and surrounding areas, also plans to reactive the No. 4 reactor at Oi on July 14.

The government has set an energy-saving target of reducing the use of electricity by up to 15 percent from 2010 levels in the summer in KEPCO's service area until the Oi reactors are back online.

About 200 protesters gathered outside the Oi plant in a face-off with riot police early Sunday, shouting "No to the restart" and beating drums, according to media reports, with numbers increasing during the morning

About 650 people had taken part in a protest rally outside the plant on Saturday night, reports said.

"If the reactor is reactivated today, other reactors will be restarted one after another," 40-year-old designer Ikuyo Hattori, who came with her two children, told the Kyodo News agency.

"We cannot accept the forcible restart when the Fukushima accident has not yet been settled," she added.

In the worst nuclear accident since the 1986 Chernobyl disaster, radiation spread over homes and farmland in a large swathe of Japan's northeast after the March 2011 disaster.

Nobody 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 protests have gathered momentum nationwide since Noda's announcement with widespread scepticism over his assertion that safety has been assured at Oi.

On Friday, tens of thousands of people swarmed on streets outside the premier's residence in central Tokyo with the turnout estimated by organisers at up to 180,000.

Police estimates were around 20,000, according to media reports.

A week earlier, an estimated 45,000 people gathered in a similar protest rally there.

Smaller-scale protests had been held every Friday outside the premier's residence since late March, and have been led in part by Nobel Prize-winning author Kenzaburo Oe, who started an anti-nuclear petition that has so far gathered more than 7.5 million signatures.