By Andrew Osborn

MOSCOW (Reuters) - Two young anti-Putin activists trudged through a snow-logged Moscow housing estate on a recent Saturday, putting up fliers promoting a boycott of a presidential election next month.

"It's not an election, it's a trick," read one, depicting a goggle-eyed caricature of Vladimir Putin, who polls show should be comfortably re-elected on March 18.

A man donning a fur hat ripped one of the fliers down within a minute. A woman, told by the activists "our elections have been stolen", quietly shut her door in their faces.

Unglamorous and at times disheartening for those involved, this is the sharp end of opposition leader Alexei Navalny's campaign to boycott an election he says amounts to the rigged reappointment of Putin, whom he likens to an autocratic Tsar.

Navalny, a 41-year-old lawyer whose protests and corruption exposes of the sometimes gilded lives of government officials have irked the Kremlin, has been barred from the contest over what he says is a trumped up suspended prison sentence.

Unable to challenge 65-year-old Putin at the ballot box and kept off state TV, he has devised a different strategy: A long-term political siege of a man most Russians consider invincible.

"We want to tear Putin down from his pedestal," Vladimir Milov, an economic adviser and one of Navalny's allies, said in an interview. "Putin will get a formal victory, but we want to make it a pyrrhic victory. We want to use the election to show that he doesn't have as much support as he claims."

Putin is credited with an approval rating of around 80 percent, bolstered by state TV, the ruling party and intense pride in parts of society over the 2014 annexation of Crimea and what the Kremlin has cast as military victory in Syria.

Nobody doubts that a man who has seldom been off Russian TV screens for the last 18 years is headed for a landslide.

"Leader of the political Olympus" was how Putin's spokesman described his boss last month, a nod to the fact that none of the seven candidates registered to run against him are a threat.

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Among them are a female TV celebrity whose father was Putin's political mentor, a pro-Putin businessman who says he's not cut out to be president, and a millionaire communist.

The contest, say critics, is a poor imitation of democracy.

LAME DUCK?

Putin has dismissed Navalny as a troublemaker bent on sowing chaos on behalf of Washington.

Navalny's immediate aim is to reduce turnout in the election by 10 percent or more with leaflets, social media, protests and telephone calls urging people not to vote.

"We understand that Putin is reappointing himself and that all we can do is to make that reappointment less convincing," Oleg Stepanov, a senior Navalny activist, who has helped coordinate campaigning in Moscow, said.

Turnout, according to official figures, was a healthy 65.25 percent in 2012 when Putin was last elected, and sources in the presidential administration have told the Russian media that the Kremlin is targeting a 70/70 scenario this time round, whereby Putin would win 70 percent of the vote on a 70 percent turnout.

Leonid Volkov, head of Navalny's campaign, says turnout was inflated in the past and will only be in the 40-45 percent range this time, noting it fell below 50 percent in the 2016 parliamentary poll for the first time in the post-Soviet period.

Navalny plans to deploy tens of thousands of observers on election day to chronicle and, he hopes, discourage fraud. Activists attending packed masterclasses in a grey Soviet-era Moscow tower-block learn how to monitor polling stations and hear warnings that "everyone" will lie to them.

"Our strategy is for Putin to turn into a 'lame duck' on March 19 (the day after the election) and into a politician who cannot be re-elected," Volkov, the campaign chief, told activists in Yekaterinburg in January.

"The authorities' actions suggest this issue (of turnout) is a painful one. Let's hit them where it hurts," he said.

LONG-TERM SIEGE

Armed with what they say will be irrefutable evidence of fraud and their principal grievance -- that Navalny was unfairly barred from standing -- Navalny's supporters plan post-election protests demanding the election is re-run and Navalny included.

The Kremlin is unlikely to acquiesce and there is a risk of violence in Moscow and St Petersburg if the authorities crack down. Officials appear reluctant to do that however, to avoid fuelling a movement they say does not pose a serious threat.

Opinion polls put Navalny's support at less than 2 percent and many Russians, who still get much of their news from state TV, say they do not know who he is.

Backers note he won almost a third of the vote in a 2013 Moscow mayoral race and say he would become the "anyone but Putin candidate" if ever allowed to run, forcing Putin to a second voting round, although Putin would then win.

The Kremlin's assessment is less generous. Putin, when asked about Navalny, makes a point of never mentioning his name, while Deputy Prime Minister Sergei Prikhodko, the target of one of Navalny's investigations, has called him "a political loser."

Many of Navalny's supporters, attracted by his YouTube videos, investigations and informal direct language, are people aged 25-35 in big cities. They say others, including former government supporters, are joining them.

"A time of change is coming," said Alexander, a 29-year-old businessman and Navalny supporter in the Urals city of Yekaterinburg who declined to give his surname.

Navalny promises genuine political competition and democracy, pledging to depoliticise the judiciary, champion a free media, promote free market economics and introduce a visa regime for visitors from Central Asia and the South Caucasus.

He talks directly to supporters through his online TV programs and now has more than 80 regional headquarters and some 200,000 volunteers. But he knows he will need a much bigger network and greater influence among Russia's 144 million people and will also have to keep campaign contributions coming in.

For now, donations average around 500 roubles ($8.87) and around half are from grassroots supporters. The other half, say sources familiar with the situation, come from a number of millionaire businessmen who are not household names.

Navalny has been jailed repeatedly for breaking draconian protest laws, says he and his family are being tracked by the security services, and his brother, Oleg, is in jail on what he says is a politically-motivated embezzlement charge.

If, as recent Russian history would suggest, he fails to convert minority anger into majority change in next month's elections, there is a long-shot Moscow mayoral election later this year and a Moscow city parliament election the next.

Further out, he will target a 2021 nationwide parliamentary election leading up to the next presidential race in 2024.

"The result will perhaps not be immediate, but it will happen," said Madina Avsetova, 19, one of the two activists who spent a recent Saturday pasting up election boycott fliers.

Svyatoslav Kotov, her 24-year-old co-activist, said they would need to work relentlessly.

"We all want things to change quickly," he said. "But we understand that such change only happens in fairytales."

(Additional reporting by Natalia Shurmina in Yekaterinburg; Editing by Philippa Fletcher)