By Nastassia Astrasheuskaya and Steve Gutterman
MOSCOW (Reuters) - Russian investigators charged street protest leader Alexei Navalny with theft on Tuesday and banned him from leaving the country, threatening a heavy jail term in what supporters say is a growing crackdown on dissent by President Vladimir Putin.
Navalny, an anti-corruption blogger who has organized demonstrations that have dented Putin's authority, dismissed the charge as absurd and other opposition leaders accused Putin of using KGB-style tactics to try to silence his critics.
Other moves which the opposition depict as a crackdown on dissent since Putin began a six-year term in May include a law increasing fines for protesters, closer controls of the Internet and tighter rules for foreign-funded campaign and lobby groups.
Russia's federal Investigative Committee said in a statement that Navalny, 36, had been charged over the theft of timber from a state firm while he was advising a regional governor in 2009, and he could face a 10-year sentence.
"I have been charged and ordered not to leave," Navalny said after emerging from the Investigative Committee headquarters, where he had been summoned for the presentation of what he had expected would be a less severe charge.
"This is really quite absurd and very strange because they have completely changed the essence of the accusation, compared to what it was before," Navalny, who had been questioned repeatedly since the case was opened in 2010, told reporters.
He made clear he would not be silenced. "I will continue to do what I have been doing, and in this sense nothing changes for me," said Navalny, a lawyer. "We believe that what is happening now is illegal. We will use the methods of legal defense at our disposal. What else can we do?"
Navalny is one of the few people seen as capable of emerging as a viable leader of the fractious opposition, although critics say he has nationalist tendencies.
He gained prominence by fighting corruption at state-controlled companies and used the Internet to do so, appealing to a tech-savvy generation of urban Russians who have turned away from the mainstream media.
Before parliamentary elections last December he helped to energize a struggling opposition, popularizing a phrase referring to the ruling United Russia party, then headed by Putin, as the "party of swindlers and thieves".
He was also among the leaders of large protests prompted by allegations of fraud in the election on behalf of United Russia, which saw its big majority in parliament cut to a handful of seats despite the accusations that it had cheated.
"This case has been fabricated from beginning to end," said Boris Nemtsov, a former deputy prime minister who is a prominent Putin opponent. "The true reason for what is happening is Putin's mortal fear of losing power ... He is wildly afraid of the opposition, including Navalny."
In a reference to the Cheka secret police, a precursor of the Soviet KGB, Nemtsov said: "Putin is using traditional Chekist methods ... Fabricated cases, charges, arrests, jail."
Putin won a presidential election on March 4 despite the largest protests since the start of his 12-year rule, during which he has served as president for eight years and as premier for four. At times attendance at the rallies reached more than 100,000, witnesses said, although they have become less frequent since Putin returned to power.
But opponents say a series of steps he has taken in recent months to tighten control show the former KGB spy is worried about losing his grip on the world's largest country.
Putin, who has repeatedly warned against rocking the boat in speeches since his election, signed a law on Monday toughening punishment for defamation and another on Tuesday that opponents say could be used to censor the Internet.
In a case which critics say will indicate how he plans to treat opponents during his new term, three women from the punk band "Pussy Riot" went on trial on Monday over an unsanctioned protest performance at the altar of Russia's main cathedral, where they called on the Virgin Mary to "throw Putin out!"
Their trial entered its second day on Tuesday in a Moscow court, and they face up to seven years in jail over a protest they say was aimed against the close relationship between Putin and the Russian Orthodox Church.
Navalny had been detained and served brief terms in custody several times over administrative offences linked to the protests, but had never been charged with a more serious crime.
Lawyers for Navalny had said on Friday they expected he would be charged over the case in Kirov province. But they had expected him to face a different charge punishable by up to five years in jail, rather than 10.
The Investigative Committee said more than 10,000 cubic meters of timber were stolen as the result of a plot between Navalny and two company chiefs, causing the regional government to lose more than 16 million roubles ($497,000).
(Reporting by Nastassia Astrasheuskaya, Writing by Steve Gutterman; Editing by Timothy Heritage and David Stamp)