* Blogger emerges as major opposition figure
* Protesters chant his slogan against ruling party
* U.S.-educated Navalny embodies Russian mood change
* Nationalism, anti-immigrant views irk liberals
MOSCOW, Dec 11 (Reuters) - If Vladimir Putin is to face a Russian rebellion, its spiritual leader may be a 35-year-old blogger named Alexei Navalny.
At Saturday's protests, the biggest of Putin's 12-year rule, some of the loudest cheers were for the anti-corruption campaigner, who has warned Russia's paramount leader he could face an Arab Spring-style revolt.
Though he was absent from the rallies, sitting in jail since a protest last week against vote-rigging in the Dec. 4 parliamentary election, Navalny is in the vanguard of a mood change among Russia's urban youth against Putin's rule.
"You cannot beat up and arrest hundreds of thousands or millions," Navalny said in a statement from jail that was read out to demonstrators on Saturday. "We are not cattle or slaves.
"We have a voice and we have the strength to defend it."
The message, issued while he serves out a 15-day sentence for obstructing police during a demonstration, was also posted on his blog at navalny.livejournal.com/ .
Navalny represents a new, Internet-savvy generation and is seen as a potential threat to Putin, even though the prime minister and former KGB spy runs a tightly controlled political system that he has crafted since his rise to power in 1999.
Asked about his own ambitions during an interview with Reuters in May, Navalny winced but his blue eyes twinkled: "I would like to be president," he said.
"But there are no elections in Russia."
With a courage that some would say borders on folly, Navalny dismissed the dangers of challenging Putin: "That's the difference between me and you: you are afraid and I am not afraid," he said.
"I realise there is danger, but why should I be afraid?"
He has no political party but Navalny has become possibly Russia's most popular political blogger by using his computer keyboard to illustrate the absurdities of a corrupt bureaucracy.
Yet his character and politics are also more complex - some might call them contradictory - than admiring Western liberals might expect of a Yale-educated lawyer who has taken to buying small stakes in some of Russia's biggest companies in order to demand greater transparency for shareholders, and the public.
While his time in the United States on a fellowship at Yale has forced him into denying accusations from Putin supporters that he is a CIA plant, his hostile views on Muslim and Asian migration into Russia's Slavic heartland have also seen him obliged to rebuff suggestions that he has "fascist" tendencies.
An outspoken Russian nationalist, he was expelled from a liberal opposition party and promises to crack down on immigration from Central Asia and the Caucasus.
His role, never fully explained, in a brawl, and alleged air pistol shooting, in 2007, adds to an edgy air of mystery around the tall, lean attorney who sets off chiseled Slavic cheekbones and piercing blue eyes with a marked taste for argyle-pattern sweaters and jeans.
'SWINDLERS AND THIEVES'
Shooting to prominence by challenging state companies such as pipeline operator Transneft to explain millions of dollars of unorthodox payments, Navalny coined the defining slogan of the parliamentary election campaign by branding Putin's ruling party a collection of "swindlers and thieves".
The United Russia party confused its response to the accusation, first with silence, then by outrage and threats and then by trying to address the slur as it began to roll off the tongues of Russians with alarming regularity.
As appealing to many Russians as bashing bankers is for some voters in the West, Navalny's phrase struck a chord with millions disgusted by the ostentatious wealth of Moscow's elite.
Navalny's words were draped on a banner over a bridge in central Moscow on Saturday when tens of thousands of people came to protest against vote-rigging. Hundreds turned to the Kremlin at one point, chanting "swindlers and thieves".
"Navalny is the only possible leader I see," a Moscow-based Western banker said of Russia's fragmented opposition.
"He has fire in those blue eyes of his."
He has also challenged Putin and the Russian establishment directly, accusing the 59-yeard-old leader of ruling a venal elite as "chairman of the board of Russia Inc".
Putin's spokesman has denied as "simply ridiculous" charges made by U.S. diplomats that Putin rules Russia by allowing an upper crust of corrupt officials and spies to siphon off cash from the world's biggest energy producer.
Opinion polls show Putin remains by far the most popular politician in Russia, and powerful businessmen say he is the ultimate arbiter between the competing clans of associates which own swathes of its industry and vast natural resources.
Though Navalny is nowhere near Putin in terms of popularity, he is tipped as a potential future leader by foreign diplomats. He has even earned the grudging respect of some Kremlin allies by mobilising a deeply divided and wilting opposition.
"He is a clever lad," said one source close to the Kremlin. "He is a talented politician."
Some of his critics even charge that he is the creature of the Kremlin spin doctors, though he laughs off such theories as paranoia. But he is less happy to speak about who funds his activities, saying merely that they do not seek publicity.
Some protesters in Moscow on Saturday held magazine covers bearing a picture of Navalny, who appeals to many of the 'IKEA generation' - middle-class young Russians who furnish their apartments with the Swedish mass retailer's wares.
Made prosperous enough to dabble in the delights of Western consumerism during the boom years of Putin's 2000-08 presidency, many young, urban Russians are fatigued by perceptions of their country's stagnation and endemic corruption.
Navalny has been able to use satire to mobilise a growing sense of disenchantment with Putin's elite while also appealing to nationalists - otherwise a natural support base for Putin - who complain that he has betrayed Russians by letting in too many non-Slavic migrants from Central Asia and the Caucasus.
But Navalny's new prominence also poses a challenges: can he really unite a disparate protest movement which includes investment bankers, nationalists, socialists, devout Russian Orthodox Christians and free-market liberals?
And even if he can do that, can he then appeal to a large part of the more than 140 million population? Uniting a divided opposition, and the people of Russia across the world's largest national territory, is a huge task.
Putin has had a tight grip on the traditional media since he came to power. Navalny, however, is a force on the Internet and the protests since last Sunday's election have shown the power of social media and blogs to mobilise large numbers of people.
After Putin blamed the United States for stoking the protests over vote-rigging, Navalny is likely to face more accusations that he is doing the dirty work of Russia's old Cold War foe. Following the campaign against Transneft, its CEO, Nikolai Tokarev, said Navalny was backed by U.S. politicians.
Navalny has denied working for a foreign power and says corrupt Russian bureaucrats are a greater potential threat to national security - because their wealth hidden abroad could open them up to blackmail by foreign intelligence services.
"I am not an agent of the CIA," said Navalny, who took a law degree at Moscow's renowned Peoples' Friendship University before studying securities trading in the city.
Though his nationalism may also give him wider electoral appeal, some of his critics warn that Navalny's liberal supporters have ignored his hardline views on immigrants.
He was kicked out of the small liberal Yabloko party after seven years as a member because of his strong nationalist opinions, said Yabloko leader Grigory Yavlinsky.
But Navalny is unrepentant, dismissing in his interview with Reuters any suggestion that it was "terrible fascism" to argue that many Russians share his concern over illegal immigration from former Soviet states in central Asia and the Caucasus.
"There is a problem," he said. "We have a huge number of migrants whose behaviour and cultural code is way out of joint with the cultural codes of those living here, the Russians."
He said keeping Chechens, Dagestanis and others from the Muslim North Caucasus provinces out of the Slavic heartland was "even more complex because they are Russian citizens". He favours cutting central government spending on those regions.
In 2007, Navalny was reported by a state news agency to have been involved in a brawl at a Moscow club. After being ejected by bouncers, he got into a fight on the street and was quoted as saying at the time that he had shot his opponent with an air pistol. Charges were later dropped.
Any doubts about his character or opinions seem to weigh little with many of Russia's anxious middle class for whom Navalny has now become a leader -- in thought, at least, if not yet in practice.
So much so that in Putin's home city of St Petersburg protesters held up a banner on Saturday reading:
"Navalny for president; the pack of swindlers to prison." (Additional reporting by Steve Gutterman; Editing by Timothy Heritage and Alastair Macdonald)