Russia's most famous prisoner, former billionaire oil tycoon Mikhail Khodorkovsky, is marking the six anniversary of his arrest with little hope for clemency from the Kremlin.

Khodorkovsky, once Russia's richest man, is serving an eight-year sentence on fraud and tax evasion charges. He said that the authorities apparently expected him to acknowledge guilt in order to receive a pardon — something he has no intention of doing.

"The current government considers "repentance" the necessary condition for pardon," Khodorkovsky said in remarks posted on the independent online newspaper gazeta.ru. "I don't suffer from excessive pride, but it's unacceptable for me to confess to crimes I never committed."

About 100 Khodorkovsky supporters rallied in downtown Moscow Sunday to push for his release.

Khodorkovsky's case has been widely seen as a punishment for challenging former President Vladimir Putin and also as a part of government efforts to strengthen the state's grip on energy resources.

Khodorkovsky's Yukos oil company, once Russia's largest oil producer, was declared bankrupt in 2006 and later sold to pay billions of dollars in alleged back taxes. Most Yukos assets have been purchased by the state-controlled Rosneft oil company in the effective renationalization.

Khodorkovsky is now facing a new trial on charges of embezzling more than $25 billion worth of oil from Yukos subsidiaries and laundering most of the proceeds.

Khodorkovsky and his supporters have denounced the new charges as a politically driven attempt to keep him behind bars for decades.

"If we were dealing with normal court procedures, the judges (would) have no choice but to cancel, to stop it," Garry Kasparov, the former world chess champion turned opposition leader, told Associated Press Television News. "But it still goes on."

Khodorkovsky said he could have gone abroad to escape arrest, but decided to stay in Russia.

He was arrested on Oct. 25 by gun-toting riot police who broke into his private jet in Siberia. He and his business partner Platon Lebedev were convicted on fraud and tax evasion in 2005.

Khodorkovsky's second trial has been seen as a test of whether Russia's justice system has changed under Putin's successor, President Dmitry Medvedev.

A lawyer by education, Medvedev vowed at his 2008 inauguration that judicial reform and the rule of law were among his priorities. Critics say, however, that he has remained in the shade of Putin, who is continuing to call the shots as Russia's powerful prime minister.

Khodorkovsky's mother, Marina, sounded increasingly desperate.

"Hopes are melting. My hopes for the new president are melting," she told APTN. "One always hopes for something, hopes for the best. But I think that in ruling for a year and a half one should be able to take some more effective steps."