Hundreds gathered near the plant in the town of Ohi to protest against the move, which has divided public opinion.

Road blocked

The reactor is expected to be fully operational by the end of the week, the operator, Kansai Electric Power Co (Kepco) says.

About 100 of the 650 protesters at the nuclear plant blocked a nearby road overnight, but a Kepco spokesman said the reactivation was not affected, according to the Reuters news agency.

The restart of the reactor follows an order by Mr Noda last month authorising the reactivation of both it and another reactor at Ohi - No 4 - following stress tests. Reactor No 4 is to be restarted on 14 July.

At the time, he called on the Japanese to support the move, saying it was needed to bolster the economy and prevent energy shortages over the summer.

The decision was welcomed by businesses who had voiced concern over the lack of power for industry.

Dissent

The BBC's Mariko Oi, in Tokyo, says it was one of the largest demonstrations seen since the reactors at Fukushima were damaged in March 2011.

Tokyo protester

Reports differ on the number of attendees, but organisers say 200,000 people took part, our correspondent says.

The government is continuing to assess whether other nuclear plants are safe to be reactivated.

But demonstrators say they are not convinced by assurances over safety. They argue that Japan should take the opportunity to move to alternative energy sources.

Tokyo-based protester Nobuhiko Shudo told the BBC the problem of disposing of radioactive waste was key.

"The most important thing for us is sustainability of the Earth for the next generation, so if we have some problems to keep the planet clean and beautiful, then we have to change the industrial structure" to foster alternative energy sources, he said.