Four weeks into a hunger strike, defeated Russian mayoral candidate Oleg Shein's cheeks are sunken and his clothes hang loosely off his slight frame.

Shein, claiming his victory was stolen in favour of a rival from Vladimir Putin's United Russia party, has vowed not to eat until a new vote is held in this provincial capital in southern Russia.

After his election to a third term as president sapped the strength of the biggest protest movement of Putin's 12-year rule, Shein's struggle has given the opposition new focus and has thrust the professorial, bespectacled 40-year-old to the centre of Russia's political fray.

It has drawn leaders of big winter protests in Moscow to this sleepy Caspian Sea city, the latest staging point in a new strategy to take their fight local and chip away at Putin's authority during a six-year presidential term that starts with his inauguration on May 7.

Shein, a former member of the State Duma lower parliament house who supporters say also was stripped of victory in a 2009 mayoral race, said the hunger strike was a last resort.

We have tried everything. This is not the first such (vote) theft in Astrakhan. We are fed up with it, he said, looking gaunt and pale in black shirt and slacks.

We have tried the courts, complaints to the prosecutors. We talked to Putin. We held protests - everything we could think of.

Shein and several supporters began the hunger strike on March 16. Some quit on the advice of doctors after losing more than 20 percent of their body weight, but others have taken their place to keep the total numbers at around 20.

Many of them clad in sweatpants and slippers, the strikers are camping out in two second-floor rooms littered with plastic cups and water bottles in an administration building, sleeping on air mattresses and using computers as a link to the outside world.

Indefinite political hunger strike against the falsification of mayoral elections in progress, reads a sign on the door to the office where Shein spends most of his time during the day, responding to a constant stream of calls from reporters and expressions of concern from supporters.

RIGHT TO CHOOSE

We are fighting for only one thing: For the right of people to choose their own government, Shein said.

Protest leaders, including popular bloggers, television personalities and satirists, say they hope to bring people into the streets for protests in a test of plans to galvanise grassroots interest in local campaigns.

We are mobilising famous people, politicians of all stripes to come here to inspire people to overcome their fear and take to the streets, Dmitry Gudkov, a national lawmaker in Shein's Just Russia party, said outside the strike headquarters.

That is the only thing the authorities are afraid of.

The small crowds at scattered rallies in Astrakhan this week suggests they face an uphill battle. While many people said they felt cheated by the mayoral poll - held the same day Putin secured his return to the Kremlin after four years as prime minister - few planned to protest.

Local media mostly had been silent about the strike, until the guest stars from Moscow arrived. While Muscovites eagerly read Shein's blog, Internet use in this city of half a million is less widespread and few people knew or cared.

Some said they feared reprisals at work or university, but most expressed apathy.

When have we ever had fair elections? How are they going to change that now? asked Sergei, a 25-year-old construction worker who gave only his first name. Everyone I know voted for Shein. Of course he won - but the authorities won't give up power.

His remarks echoed the widespread disbelief in the official results of a December parliamentary election that drew tens of thousands of people to protests in Moscow and smaller crowds in other cities.

The Kremlin has endorsed limited political reforms in response to the protests, allowing more parties and promising governors will be elected, not appointed, but Putin rejected the protesters' main call for a re-run of the parliamentary vote.

In similar style, Putin poured cold water on Shein's fight in a televised appearance in parliament on Wednesday, calling his decision to go on a hunger strike strange and saying only the courts could resolve his election grievance.

That call seems disingenuous to Kremlin opponents who say the courts lack independence and usually rule against election challenges.

Still, even the discussion of the hunger strike in parliament - seen as a politics-free rubber stamp for Kremlin initiatives for much of Putin's rule - is a sign of change.

Several Just Russia deputies walked out of the State Duma lower house to protest Putin's response.

While the winter protests failed to prevent Putin from winning the presidency, they dented his air of invincibility and undermined the party he has long used as both a source of support and an instrument of power.

United Russia lost 66 State Duma seats in the December vote, leaving it with only a slim majority.

Inspired by recent losses for United Russia in mayoral votes, including in Yaroslavl on April 1, Kremlin opponents are preparing to contest elections in the Siberian city of Omsk in June and other cities and regions in the autumn.

Society has changed. United Russia can no longer act without conscience like it did before. I think United Russia is a dying party, Shein said. It is very important to make something fresh and new out of its decomposing remains.

WEB CAMERAS

In Astrakhan , official election results gave about 60 percent of the vote to United Russia candidate Mikhail Stolyarov, a businessman who served as deputy to the previous mayor, while Shein received about 30 percent.

There were violations before, but never on such a scale and before we didn't have cameras, said Sergei Kozhanov, an activist blogger who has studied the results. He quit the hunger strike on Tuesday after dropping 10 kg (22 lbs).

Shein won a small victory when authorities agreed to hand over web camera footage from polling places.

In what some experts say is a sign of fraud, Shein won by a comfortable margin at most polling stations where automated ballot boxes were installed, while he got around 2 percent of the vote at many stations where ballots were hand-counted.

Dozens of videos posted online show observers being roughly kicked out of polling station during the vote tallies.

Both sides are standing their ground in the stalemate. Regional governor Alexander Zhilkin has flatly refused to call a new election, citing a report by prosecutors he said showed Shein's claims were baseless.

As a local legislator, Shein has won respect from activists who see him as a moral authority, walking to work and eschewing the black SUVs and expensive suits favoured by many politicians.

Many activists from other parts of the country see his battle as an extension of their own, and plan to travel to Astrakhan for protests on Saturday.

Putin created the problem in Astrakhan because his people here were involved in colossal falsifications, said Alexei Navalny, a popular anti-graft blogger and opposition leader.

The situation will heat up and people will come here. Shein is running the show: His life is threatened but he won't give up, he said.

Either we make them call a new election or we can't do this. That's all.

(Reporting By Alissa de Carbonnel; Editing by Steve Gutterman and Michael Roddy)