The European Court of Human Rights ruled on Tuesday that Mikhail B. Khodorkovsky, the imprisoned Russian oil tycoon, had been illegally arrested and held under inhumane conditions. But it also found that Khodorkovsky had not proved that his prosecution on charges of fraud and tax evasion was politically motivated.

“The Court admits that the applicant’s case may raise a certain suspicion as to the real intent of the authorities,” the decision said. The court added, however, that it required incontrovertible proof to find that the entire legal machinery of the state had been misused against Khodorkovsky.

Khodorkovsky was convicted on the charges and sentenced to eight years in prison.

The decision was bound to please Russian authorities, who have contended the case was purely a matter for the courts. But it was unlikely to change the widespread view among human rights organizations and Western critics that the billionaire was indeed prosecuted because he financed opposition to Vladimir V. Putin, who was president when Khodorkovsky was arrested in October 2003.

Human Rights Watch and the Moscow Helskinki Group denounced the ruling Tuesday. “It is difficult for me to say which argument exactly would be deemed irrefutable by the court,” said Tatyana Lokshina, deputy director of HRW’s Moscow office. “From our point of view, the politicization of this case is obvious.”

Former Yukos CEO Mikhail Khodorkovsky stands behind a glass wall, with his wife Inna reflected in the glass, in a courtroom in Moscow, Russia, Tuesday, May 24, 2011. (Misha Japaridze/AP)

Lyudmila Alexeyeva, head of the Moscow Helskinki Group, told the Interfax News Agency: “We see the political motive, and they don’t. They should put their glasses on or get a magnifying glass.”

In February 2004, Khodorkovsky filed a complaint about his arrest and treatment with the court, which rules on violations of the European Convention on Human Rights, which Russia has signed. The court, based in Strasbourg, France, leaves enforcement to the governments involved.

The court agreed that Khodorkovsky had been illegally arrested when armed police stormed a Siberian airport where he had flown on business and took him back to Moscow, saying he was needed as a witness. Instead, the police quickly presented him a 35-page list of charges. It was clear, the court said, that Khodorkovsky had been wanted not as a witness but as a defendant.

The court also agreed that Khodorkovsky was held in inhuman and degrading conditions when he spent two months in a 43-square-foot space with “appalling” sanitary conditions and was kept in a cage throughout his trial, even though he had no criminal record and there was no evidence that he was violent.

Khodorkovsky was awarded the equivalent of $35,000 in damages and expenses — the amount he requested. His attorney, Karinna Moskalenko, said it is now up to the Russian Supreme Court to decide how to enforce the ruling. Russia has three months to appeal the decision.

Khodorkovsky was found guilty last year on a second set of charges — of stealing oil from his Yukos Oil company. He was sentenced to an additional prison term, and his release is now set for 2016.

On Monday, he and his co-defendant in the oil case, Platon Lebedev, filed a parole request in a Moscow court, saying they did not admit guilt but had served half of their sentences and so were eligible for release.

Speculation arose that the authorities might grant the request after NTV television on Sunday broadcast a news item about the case and Khodorkovsky’s expected request for parole. Television is closely controlled by the government and rarely mentions the defendants or their appeals.