* Guriev leaves country after Yukos questioning
* Key adviser to PM Medvedev
* Publicly sympathised with opposition protest leader
A prominent economist and government adviser has fled Russia after being questioned by state investigators, amid a growing clamp-down on groups and individuals critical or independent of President Vladimir Putin.
Sergei Guriev, an English-speaking economist well known to Western investors, had been questioned as a witness in an investigation into the defunct Yukos oil company, whose founder Mikhail Khodorkovsky was jailed in 2005 for fraud.
His real transgression, supporters and commentators say, was to support Alexei Navalny, an anti-corruption campaigner who led protests against Putin's return to the presidency which the Kremlin has been trying gradually to snuff out.
"He (Guriev) defended Khodorkovsky and said that the case was fabricated. An enemy? Of course," Boris Nemtsov, a protest and opposition leader, said ironically. "He fights corruption? That betrays our fundamental ideals!"
Since Putin's return to the Kremlin after four years as prime minister, the authorities have moved across a broad front to silence critics, with Navalny now on trial on fraud charges which he says are trumped up and politically motivated.
Non-governmental organisations such as rights groups, independent vote monitors and opinion pollsters that receive foreign funding have been told to register as "foreign agents", a term with overtones of the Cold War and treason.
Guriev, the 41-year-old rector of Moscow's New Economic School, has advised Prime Minister Dmitry Medvedev, served on the board of state bank Sberbank and is a leading commentator in the local and international press.
The flight of the liberal economist, a former visiting professor at the U.S. university of Princeton, was confirmed when Sberbank issued a statement saying he had declined to seek re-election to its board.
Sources familiar with the situation said he left Moscow in early May to join his family and was now in France. He had tendered his resignation as head of the NES, a post he has held since 2004, but his resignation has not yet been accepted.
Guriev, reached by Reuters, said his decision was personal, that he was "on vacation" and that he would not comment further.
"This is a very bad signal, definitely," said Sergei Aleksashenko, another leading Moscow economist.
"He was the economist who was most affiliated to the government. The signal I read is that Medvedev is not willing, or able, to defend his supporters."
Erik Berglof, chief economist at the European Bank for Reconstruction and Development, said Guriev - a personal friend - had made a huge contribution to the policy debate in Russia as a speechwriter for Medvedev and an advocate of economic reform.
"LET HIM LEAVE"
The Kremlin denies cracking down on opponents or using the judiciary for political ends. Putin has laughed off accusations that he is tightening the screws on opponents.
"If he wants to leave, let him leave. If he wants to return, let him return. It's his personal affair," Putin's spokesman, Dmitry Peskov, said.
Another official who declined to be named said there would be no impact on the investment climate - growth in investment has halted amid high capital outflows - but "another small log will be thrown on the furnace of anti-Russian hysteria".
Putin critics saw Guriev's flight as another sign of pressure on economic reformers, who are being marginalised.
The Investigative Committee, Russia's most powerful law enforcement agency, said it had questioned Guriev in an unspecified investigation into Yukos, the oil firm that was bankrupted, broken up and sold off, mainly into state hands.
Guriev had previously given a critical expert opinion, said a source familiar with the matter, adding that he believed a new case was being prepared against Khodorkovsky.
Critics say the Investigative Committee is being used by a powerful faction of 'siloviki', or men of power who share Putin's background in the security services, to remove liberals from positions of power or influence.
It was Guriev's institute that provided a stage for Barack Obama to deliver a keynote address during the U.S. president's visit to Moscow in 2009 at the height of a 'reset' in relations.
Ties have since been blighted by rows over issues including Syria, human rights and a ban on the adoption of orphaned Russian children by American families.
Guriev had announced in a blog post last May - a week after Putin was sworn in - that he had donated 10,000 roubles ($320) to an anti-corruption project set up by Navalny. Navalny fears he will be jailed for 10 years but says protests, although they have now dwindled, will continue against Putin.
Guriev told Reuters last September that Putin could fall "overnight" if he fails to stamp out corruption and wean Russia off its economic dependency on oil and had questioned the fairness of recent elections.
"We all lived through the Soviet Union, which was forever until it was no more," he said.
Guriev joins other independently minded public figures, including the founder of Russia's largest online social network VKontakte, Pavel Durov, in leaving Russia under duress.
Some commentators said the departures could mark the start of a wave of emigration by Russia's best and brightest, like those in the 1970s and after the 1917 Bolshevik revolution.
"They have gone pretty far, shifting from manipulative means to repressive ones," said Maria Lipman, an analyst at the Carnegie Moscow Center. "It used to be that politics was banned. Now we are experiencing a ban on autonomy in the public space. To use Putin's metaphor - we will call the tune."