* Crowd breaks through barriers
* Defeated candidate wanted no nuclear power by 2020
TOKYO, July 29 (Reuters) - Tens of thousands of people protested against nuclear power outside Japan's parliament on Sunday, the same day a proponent of using renewable energy to replace nuclear following the Fukushima disaster was defeated in a local election.
The protesters, including old-age pensioners, pressed up against a wall of steel thrown up around the parliament building shouting, "We don't need nuclear power" and other slogans.
On the main avenue leading to the assembly, the crowd broke through the barriers and spilled onto the streets, forcing the police to bring in reinforcements and deploy armoured buses to buttress the main parliament gate.
The protest came as results from rural Yamaguchi showed that Tetsunari Iida, an advocate of renewable energy to replace nuclear power, lost his bid to become governor to a rival backed by the opposition Liberal Democratic Party (LDP), which promoted nuclear power during its decades in power, Kyodo news agency reported, citing exit polls.
Iida, who wants Japan to exit nuclear power by 2020, had promised to revitalise Yamaguchi's economy with renewable energy projects and opposed a project by Chubu Electric Power Co to build a new nuclear plant in the town of Kaminoseki.
Energy policy has become a major headache for Prime Minister Yoshihiko Noda, who less than a year in office is battling to hold his Democratic Party together before a general election due next year but which could come sooner.
Weekly protests outside Noda's office have grown in size in recent months, with ordinary salary workers and mothers with children joining the crowds.
On Sunday, the protesters - holding candles as darkness fell on the hot summer day - took their demonstration to parliament.
Chanting "oppose restarts", they pressed against steel barriers erected around the parliament building, where thousands of police were deployed to keep the peace.
Many of the crowd had marched past the headquarters of Tokyo Electric Power Co, the company at the heart of the worst nuclear crisis since the Chernobyl disaster in 1986.
"We are here to oppose nuclear power, which is simply too dangerous," Hiroko Yamada, an elderly woman from Saitama prefecture near Tokyo, said.
"(Noda) isn't listening to us. He only listens to companies and Yonekura," she said, referring to Hiromasa Yonekura, the chairman of Japan's biggest business lobby.
An upset victory by Iida, 53, would have added to Noda's woes as the government tries to decide on an energy portfolio to replace a 2010 programme that would have boosted nuclear power's share of electricity supply to more than half by 2030.
Still, Iida's support from volunteers in the conservative stronghold bodes ill for the Democrats and the LDP, support for which has failed to benefit greatly from Noda's woes, Kyodo said in an analysis of the local vote.
"The brave battle by Iida, who sought a change in energy policy, can be said to be proof the popular call to exit nuclear power has spread even to Yamaguchi," the news agency said.
Noda, who approved the restart of two idled reactors this month, has said he would decide on a new medium-term energy plan in August, although media reports over the weekend said that decision could be delayed.
Experts have proposed three options: zero nuclear power as soon as possible, a 15 percent atomic share of electricity by 2030, or 20-25 percent by the same date compared to almost 30 percent before the Fukushima disaster.
Under pressure from businesses worried about stable electricity supply, Noda has been thought to be leaning toward 15 percent, which would require all of Japan's 50 reactors to resume operations before gradually closing older units.
The growing anti-nuclear movement, however, may make that choice difficult, some experts said.
Multiple inquiries into the March 11, 2011 nuclear crisis, in which a huge quake-induced tsunami devastated the Fukushima plant, causing meltdowns and forcing mass evacuations, have underscored the failure by authorities and utilities to adopt strict safety steps or disaster response plans. (Reporting by Linda Sieg and Aaron Scheldrick; Editing by Joseph Radford and Michael Roddy)