A defector from Vladimir Putin's United Russia has been elected mayor of the city of Yaroslavl in a setback for the governing party and a triumph for the opposition's new strategy of chipping away at his rule in local elections.

Pothole-ridden roads, utility bills, childcare and housing were voters' chief worries as they gave Yevgeny Urlashov victory over a businessman on Sunday with nearly 70 percent of votes in this picturesque industrial city of 600,000 northeast of Moscow.

But opposition leaders and political experts saw the vote as a test of the staying power of the highly centralised political system that Putin has built since rising to power in 2000 and is likely to continue when he takes over as president in May.

I have never seen so many cameras in my life... I want to thank all the people who believed that changes can start in Yaroslavl, Urlashov, 44, told reporters.

People want a gulp of fresh air.

Demonstrations in big cities including Moscow, sparked by anger over allegations that fraud helped United Russia win a parliamentary poll in December, failed to prevent Putin winning back the presidency but have removed his air of invincibility.

Opposition leaders now hope to make their presence felt by winning local and regional elections and taking advantage of limited Kremlin reforms that will make it easier to register political parties in a concession to the protesters.

Stability is over (for Putin): Regional elections will be used as a testing ground for developing tactics to oppose Putin and United Russia, said sociologist Sergei Belanovsky, a director at the Moscow-based Centre for Strategic Research.

Where the opposition wins a majority, they can afford to be disloyal.

ROAD TO THE KREMLIN?

Urlashov is not a new face for local voters.

An eight-year veteran of the city council and former member of United Russia, he joined the budding protest movement in September and campaigned against graft and a lack of change in leadership since Soviet times.

He will replace a man who had held the post for 22 years and tried to help Urlashov's rival, Yakov Yakushev, by appointing him acting mayor after the first-round of the election.

The current mayor made me a proposal: He said 'Try it out, show yourself off,' said Yakushev, who owns a paint factory among other businesses.

The move backfired. Yakushev, who also ran as an independent candidate, won less than 28 percent of the votes.

We are working to ensure that the mayor is elected and not appointed, said local vote observer Andrei Novozhilo, 29, after allegations of fraud emerged. We have never been allowed to elect an honest man before.

Others have bolder long-term goals.

The road to the Kremlin lies through Yaroslavl, opposition politician Vladimir Milov wrote in a blog. The opposition showed it can take control of big cities with strategic significance. Up next: Moscow and St Petersburg.

But some are sceptical the sudden interests - evidenced by hundreds of volunteers travelling from Moscow to join local activists in monitoring the polls - will lead to real change.

There has been an upsurge of civil activity, but I think it is a way to show, 'we are doing something,' and not a real active, adequate, substantive participation, said analysts Alexandre Sokolov of the Yaroslavl State University.

It is not clear whether, how this upsurge will end, maybe it will end in apathy.

DYING PARTY

Yaroslavl did not witness protests on the scale of those in Moscow and other cities such as St Petersburg. But support for United Russia was the lowest nationwide in the December parliamentary election.

Anger runs deep in Yaroslavl, where many residents blame the authorities for a plane crash that killed almost all the players in the city's ice hockey team, Lokomotiv Yaroslavl, last September.

The team had been forced to change the venue of a match and fly to Minsk in Belarus because a conference, hosted by President Dmitry Medvedev, was being held in its own stadium.

Many cars sport bumper stickers commemorating the players that were killed and fresh graffiti on the road into town says We remember under the team's emblem.

If it hadn't been for this conference, all of our boys would have been healthy now and playing at home, 22-year-old student and ice hockey fan Ilya Gagin said.

Nothing is being developed here, he said. New roads aren't built, just patched up and completely destroyed two-three months later. People don't know where their money is going.

United Russia's losses in other regional elections, including in industrial strongholds which have traditionally backed Putin such as the car-making city of Togliatti, has shown cracks in his rule.

United Russia is a dying party. Sooner or later it will leave the political field, Urlashov said. I am sorry that such a strong party has turned into some kind of caricature.

In regions where opposition politicians win a strong foothold, the Kremlin could find it hard to win approval for its preferred candidates to be governor or to enforce policies.

What happened in Yaroslavl is an alarming signal for the Kremlin which can't be discounted as some kind of anomaly, said Nikolai Petrov of the Moscow Carnegie Centre think-tank.

The Kremlin uses the party as a tool which helps it achieve its elections goals and this model is in crisis now ... Putin is faced with the problem of how to secure the survival of the political system.

BOOST FOR PROTEST MOVEMENT

In search of a new cause since Putin's victory in the March 4 presidential election dampened protesters' spirits, anti-Kremlin activists see Urlashov's victory as their own.

Urlashov was not the only winner, billionaire Mikhail Prokhorov, who ran as a liberal candidate against Putin, wrote on a blog on Monday, hailing activists who monitored the vote.

More and more centres of political activity are emerging, and large cities are passing on the baton. The protest movement is finding its way in concrete action.

Hundreds of volunteer vote monitors travelled 260 km (160 m) from Moscow to Yaroslavl, and some came from further afield, setting their goal as guarding against fraud at all 270 local polling stations.

The atmosphere was festive, with activists handing out white ribbons, a symbol of the protest movement, to commuters on an early morning train from Moscow.

A new movement has formed: A roving band of vote monitors, said Olga Ugnenko, 29, a computer programmer from Moscow.

The opposition is looking ahead to gubernatorial elections in the autumn and local legislative polls next year, hoping to keep denting Putin's legitimacy as he begins a six-year term.

If we see good candidates there, we will get involved and help out with their campaigns, said Georgy Alburov, an activist with a vote-monitoring project created by anti-corruption blogger Alexei Navalny.

This is training for the new presidential elections which will come sooner or later, he said.

(Additional reporting by Denis Sinyakov and Denis Dyomkin; Writing by Alissa de Carbonnel; Editing by Timothy Heritage and Ralph Boulton)