Engineers in Japan on Sunday began refiring an atomic reactor, despite growing public protests in the aftermath of meltdowns at Fukushima, ending nearly two months in which the country was nuclear-free.
Local media reported that the process to restart Unit No. 3 at Oi in western Japan began around 9:00 pm (1200 GMT) with Kyodo news agency later reporting that the reactor reached criticality early on Monday morning.
It had earlier been reported that control rods that have prevented an atomic reaction would be removed and fission would begin.
A noisy demonstration near the power station that had begun earlier in the day was continuing, live streamed footage showed.
"We are against the restart," protestors chanted to the beat of drums as they faced a line of riot police.
It was not possible to tell from the footage on http://www.ustream.tv/channel/iwakamiyasumi the size of the protest or of the police presence, however, a counter on the website showed around 25,000 viewers at 10:00 pm (1300 GMT).
Sunday's demonstration was the latest in a line of increasingly vocal anti-nuclear demonstrations in a country with little recent history of large-scale public protest.
Japan 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 Yoshihiko 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.
Sunday's protest followed a rally outside the plant on Saturday night by around 650 people, reports said.
"If the reactor is reactivated... other reactors will be restarted one after another," 40-year-old designer Ikuyo Hattori, who came with her two children, told Kyodo.
"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 of the nuclear restart, an AFP journalist at the scene said.