* Opposition sees 100-day campaign as window of opportunity

* Volunteers to hit voters with cold-calls and online posts

* Navalny camp says Putin will win but he is vulnerable

LONDON, Dec 10 (Reuters) - With his hands on all the levers of power in Russia, Vladimir Putin appears unbeatable in a presidential election but opposition activists scent a chance to show he is vulnerable.

Putin declared his candidacy for another six-year term on Friday and there is no serious rival to challenge him, with opposition leader Alexei Navalny serving more than 30 years in prison and other critics also jailed or in exile.

With the Kremlin in full control of state media and able to decide who can and cannot run, the Navalny camp says this is not a real election. But it sees the 100-day campaign window as a rare opportunity to draw Russians into a political conversation and convince them that the Ukraine war and the economic strains it has brought are problems of Putin's making.

"Of course it's impossible to beat Putin in the 'elections'," top Navalny aide Leonid Volkov told Reuters. "The aim of our campaign is to change the political agenda in Russia."

The Kremlin says Putin will win another six-year term because he commands overwhelming support across Russian society, with opinion poll ratings of around 80%.

So far only three people have declared their intention to run against him. Two are low-profile figures, Boris Nadezhdin and Yekaterina Duntsova, who may struggle to gather the 300,000 signatures required to support their candidacies. The third, nationalist Igor Girkin, is in jail awaiting trial on a charge of inciting extremist activity.

Other possible candidates who have yet to declare include Communist leader Gennady Zyuganov and liberal Grigory Yavlinsky, both political veterans and serial election losers.

'NOT PUTIN'

With the line-up still unclear, the Navalny camp has launched its campaign by simply urging Russians to vote against the incumbent.

"We don't have our own candidate. We had a candidate, Navalny, and they refused to register him, tried to kill him and put him in prison. Now we have, so to speak, a collective candidate 'against Putin'," said Lyubov Sobol, a close Navalny associate who, like Volkov, is on an official list of "terrorists and extremists" and is now based outside Russia.

Navalny's supporters cast him as a Russian version of South Africa's Nelson Mandela who will one day be freed from jail to lead the country.

Russian authorities view Navalny and his supporters as extremists with links to Western intelligence agencies intent on trying to destabilise Russia. Putin has warned the West that any meddling inside Russia will be considered an act of aggression.

The opposition is seeking volunteers from among the hundreds of thousands of people who have fled Russia since the start of the war and asking them to cold-call voters - ideally as many as 100 each, Sobol said in a telephone interview.

Many people would be scared and put the phone down, but others could be persuaded to talk, she said.

On its website NePutin (NotPutin).org, the Navalny team also calls for volunteers to spread videos and campaign messages online, and to stick up leaflets and scrawl graffiti in the streets - what Sobol described as "partisan" tactics.

"Putin's task is to make sure these elections go as smoothly and calmly as possible, without any strain on the nerves. Our task is the opposite," she said.

Within hours of parliament announcing the March 17 election on Thursday, the Navalny camp had fired its first campaign shot. It posted photos on social media of giant blue billboards it had placed in major cities, with an innocent-looking new year greeting to Russians. Underneath was a QR code leading to the NotPutin website.

The stunt showed the ingenuity of Navalny's tech-savvy team. But the impact was brief, as authorities took down the billboards and blocked access to the site.

'POINTS OF TENSION'

The opposition has struggled in the past to present a united front between the Navalny camp and other prominent Putin opponents such as former oil tycoon Mikhail Khodorkovsky and former world chess champion Garry Kasparov.

From outside Russia, all of them face a huge task to mobilise people in a country where nearly 20,000 have been detained since Russia's February 2022 invasion for protesting against the war in Ukraine.

Sobol said opposition polling showed many people were dissatisfied with Putin but were afraid to speak out. She said it was precisely the degree of repression in Russia that showed that the authorities are worried.

"Putin is in a vulnerable position. We on the side of the opposition must create more points of tension for him," she said.

"We must make problems for him and his regime, and agitate, put out counter-propaganda and tell the truth."

(Reporting by Mark Trevelyan. Editing by Jane Merriman)