All of Japan’s 50 functional reactors were taken offline one by one for safety checks after the disaster at the Fukushima Daiichi plant, which contaminated a large part of northern Japan with radiation.

Image The Ohi reactor is expected to generate power by midweek. Credit... The New York Times

The last reactor was shut down in early May amid widespread public concern over the safety of nuclear plants in the event of another large earthquake and tsunami of the sort that struck Japan in March 2011.

Mr. Noda said the restarting of the reactor was necessary to avoid crippling power shortages in the heavily urbanized Kansai region, which the plant serves. However, his decision has drawn unusually vocal public opposition in normally compliant Japan, with many Japanese saying that he ignored safety concerns to protect the powerful nuclear industry.

Over the weekend, about 200 protesters blocked the road to the Ohi plant. Kansai Electric said it had enough staff members in the plant to restart the reactor, but a senior vice minister from the ministry in charge of nuclear power had to be ferried to the plant by boat.

“I’m watching this with a tense feeling,” the official, Seishu Makino of the Ministry of Economy, Trade and Industry, said of the restart, according to Japanese news reports. “The government has taken a necessary step forward despite controversy that has divided the nation.”