Engineers were Sunday readying to refire a nuclear reactor, despite growing public protests in the aftermath of meltdowns at Fukushima, ending nearly two months in which Japan was nuclear-free.

Control rods that have prevented an atomic reaction inside Unit No. 3 at the Oi plant in western Japan were set to be removed from around 9:00 pm (1200 GMT). The reactor was expected to reach criticality nine hours later.

Hundreds of protesters blocked the road to the front gate of the power station, according to media reports, the latest in a line of increasingly vocal anti-nuclear demonstrations.

The country has been operating without nuclear power since early May when the last of its 50 working reactors was shut down for a scheduled safety check.

Restarts had put been on hold as the government mulled its options following a 9.0-magnitude earthquake and tsunami that crippled reactor cooling systems at Fukushima while more giant quakes were forecast to strike Japan.

But on June 16, Prime Minister Yoshihito Noda gave the green light to restart two reactors at the Oi plant in an effort to head off a summer power crunch amid warnings of a huge electricity shortfall.

Prior to Fukushima, nuclear power had supplied a third of Japan's electricity needs.

Oi operator Kansai Electric Power Co. (KEPCO) said the 1.18 million-kilowatt reactor would begin generating power on Wednesday and would be operating at capacity in a week.

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

A series of minor glitches have been reported at the plant since June 16 with alarms activated at locations including where the plant receives external power supply and on equipment used to monitor power transmission lines.

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 Sunday, shouting "No to the restart" and beating drums, according to media reports.

Around 650 people had taken part in a 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 pledge that safety has been assured at Oi in an area riddled with seismic faults.

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

Police estimates were around 20,000, according to media reports. No explanation was given for the large disparity.

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.

On Saturday around 1,000 people marched through the streets of Shinjuku, a major shopping and entertainment district, in protest at the nuclear restart, an AFP journalist at the scene said.