Russia on Saturday released a physicist who spent eight years in a Siberian prison on charges of spying for China in what supporters maintain was a wrongful conviction motivated by Soviet-style paranoia.

Professor Valentin Danilov was released before dawn from prison number 17 in the Siberian city of Krasnoyarsk, in an early release with over three years of his original sentence remaining, Russian news agencies said.

Danilov, 66, was first arrested in 2001 and sentenced in 2004 to 14 years in prison -- later reduced to 13 years -- for spying for China and embezzling 466,000 rubles ($15,000) from a state university to assist his work.

The criminal case against him was opened at the start of ex-KGB agent Vladimir Putin's first Kremlin term, and for many represented a return to the paranoid hunts for traitors in Soviet times.

After his release, Danilov said he hoped to return to science and that he would move back to the research town of Akademgorodok outside Siberia's biggest city Novosibirsk to live with his wife.

"Being a scientist is my way of life. Otherwise what am I for? My head cannot think otherwise," he told a news conference in Krasnoyarsk, excerpts of which were shown on state television.

Dressed dapperly in suit, tie and waistcoat, Danilov said in prison he had developed theories both in physics and also on how to improve Russia's notoriously tough prison system.

Danilov is technically on parole and will still have to report regularly to police. But he vowed to seek to clear his name and take his case to the European Court of Human Rights.

"I have still never been told what secret it is that I possess," Danilov said, quoted by the RIA Novosti news agency.

The details of his release broadcast by state television resembled a special operation -- Danilov walked out of the jail before sunrise, was dropped off outside a factory on a main ringroad where a waiting taxi took him to the city centre.

Danilov, a former director of the thermo-physics centre at Krasnoyarsk State University, was convicted of selling state secrets to a Chinese engineering company.

In a hugely controversial process, he was initially acquitted by a jury in 2003, but the verdict was then quashed by the Supreme Court and he was rapidly sentenced to 14 years in jail. He had also been held in pre-trial detention from 2001-02.

The case focused on his signing in 1999 of a contract with a Chinese engineering firm to build appliances to simulate the impact of a space environment on satellites.

Danilov always insisted the research involved had been declassified by Russia after the fall of the USSR and was available in the public domain.

Pro-Kremlin NTV television said that the information had allowed the Chinese to speed up their own satellite programme by 15 years.

Several top Russian scientists have defended his cause, saying he handed over no state secret and that his work with foreigners is normal practice in post-Soviet Russia as cash-strapped scientists try to make ends meet.

Showing a wry sense of humour, Danilov said would steer clear of the space industry in his future scientific work.

"I will work in science but not space because everything (in Russia) to do with space is always (seen as) a state secret," he said.

At the beginning of the 2000s, several experts were accused of spying by the FSB and subsequently jailed.

Arms control and nuclear weapons specialist Igor Sutiagin in 2004 was sentenced to 15 years in a labour camp, accused of having passed nuclear secrets to Britain.

He was eventually released as part of a swap between Moscow and Washington in 2010 but had always protested his innocence.

Campaigners Human Rights Watch cited both cases as examples of what it called "spy mania", in which it said suspects were tried and convicted on very little evidence and without regard to their right to a fair trial.