* Economist's flight increases fears of widening crackdown
* Putin tries to tighten grip on power after protests
* Speculation grows about new trial against ex-oil tycoon
A widening clampdown on groups and individuals critical or independent of Russian President Vladimir Putin has left his opponents asking: "Who's next?"
Liberal economist Sergei Guriev's flight from Russia under pressure from state investigators has deepened the sense of alarm as the Kremlin broadens a drive to stifle dissent and quell protests that began in December 2011.
Curbs on demonstrations, criminal cases against protest leaders and tough new funding rules for non-governmental organisations smack of the repression that accompanied stagnation under Soviet ruler Leonid Brezhnev in the 1970s, say critics.
Opponents say Putin is using strong-arm tactics to rebuild his authority, dented by the mass street protests, and note he has also sidelined aides who have fallen out of favour, including his political strategist Vladislav Surkov.
"Anyone dreaming of fighting (Putin) will be subject to the most severe measures," said Sergei Aleksashenko, a former deputy finance minister.
Resistance by Russia's elite would be crushed by a return of the communist-era choice - emigration or repression - he said, adding: "Everything will be built on his fears, manias and visions."
The former KGB spy portrays himself as the man who restored order after the chaos of President Boris Yeltsin's rule following the collapse of the Soviet Union in 1991.
He denies cracking down on opponents and has made clear he thinks there is again a need to restore order, after the biggest protests since he first rose to power in 2000.
But his critics say he seems intent on wiping out the last remnants of a protest movement that has been dwindling for months even though it poses little or no immediate threat.
Parliament has passed a series of laws which critics say he can use to undermine opponents, and several opposition figures face what they say are trumped-up criminal charges, including protest leader Alexei Navalny.
Even groups not part of the opposition are under pressure, including vote monitor Golos, which has gathered evidence of electoral fraud, and the Levada Center polling group, which has tracked Putin's falling popularity.
Both are resisting registering as "foreign agents" - a tag that has overtones of treason and echoes of the Cold War - under a new law that tightens checks on foreign-funded non-governmental organisations involved in "political activities".
Opposition figures often compare Putin's new term with that of Brezhnev's rule from 1964 to 1982. His face, imposed on a portrait of the Soviet leader - prematurely aged and decrepit - has for critics become emblematic of Putin's lengthy tenure running Russia.
Guriev's flight is seen by many as confirmation that the Kremlin has opened a new front in the war against dissenters of all stripes by putting pressure on intellectuals it mistrusts.
Behind such moves they see the influence of the "siloviki", Putin's former colleagues in the security services who are winning out as the influence of relative liberals wanes.
Guriev, a 41-year-old economist and government adviser, appears to have come under scrutiny after criticising Putin and making the cardinal sin of donating a small cash sum to Navalny.
His chances of remaining in favour were also compromised after he warned Putin could be ousted if he failed to tackle widespread corruption and reduce Russia's dependency on oil, which leaves it vulnerable to any fall in the price of its main export commodity.
NEW KHODORKOVSKY TRIAL?
Guriev's decision to join his family in France followed questioning by state investigators in a case involving the defunct oil company Yukos, once owned by a jailed former oligarch, Mikhail Khodorkovsky.
This has fuelled speculation that the next stage of the clampdown will be a new trial of Khodorkovsky and his partner Platon Lebedev, who are due for release form jail neat year.
"Their terms are coming to an end and there are people who are afraid to see them free," said Marina Khodorkovskaya, the mother of the tycoon who was once Russia's richest man but fell out with Putin after taking an interest in opposition politics.
Khodorkovsky is considered by some analysts to be of the few people who might have the stature to challenge Putin.
In a country which this year put a dead man on trial by prosecuting Sergei Magnitsky, a campaigning lawyer who exposed high-level corruption, few would rule out a new trial that could keep Khodorkovsky behind bars.
Khodorkovsky and Lebedev were arrested in 2003 and convicted in 2005 of fraud and tax evasion. They were also convicted of stealing oil from Yukos and money laundering in a second case in 2010. They are due for release next year.
Most political analysts expect Putin to see out his six-year term until 2018, even though his popularity has fallen from the peaks it hit in his first spell as president from 2000 to 2008. But he risks a backlash by putting pressure on intellectuals.
Fifteen economists and academics warned the government in an open letter on Thursday that the "foreign agents" law could threaten their independence and funding, eventually having an impact on an economy that could sliding into recession.
"There has already been a period in our history when economic science and economic analysis was fully controlled by the state," they wrote. "It is well known how it turned out for the Soviet economy."