Japan has restarted its first nuclear reactor since the meltdown at the Fukushima power plant last year.
All 50 nuclear plants there were shut after an earthquake triggered a tsunami on 11 March 2011.
It will take until Wednesday before the No 3 reactor at Ohi, in Fukui prefecture on the west coast, can start supplying electricity.
Kansai Electric Power Co says the reactor is expected to be fully operational by the end of the week.
Prime Minister Yoshihiko Noda said last month that a return to nuclear power was essential for the economy. He authorising the reactivation of two reactors.
The second reactor, No 4 at Ohi, is to be restarted on 14 July.
Hundreds gathered near the plant in the town of Ohi on Sunday to protest against the move.
The BBC reports an estimated 200,000 took part in anti-nuclear rallies outside the Prime Minister's official residence in Tokyo on Friday. They want Japan to move to alternative energy sources.
Nuclear power used to provide one third of Japan's electricity.