Mikhail Khodorkovsky has resigned as chief executive of Yukos, the Russian oil giant at the centre of a confrontation between big business and the Kremlin.
Mr Khodorkovsky said he would concentrate on charitable work
The resignation came days after Mr Khodorkovsky, thought to be Russia's richest man, was jailed on charges of fraud and tax evasion.
In a statement, he said his aim for the next few years had been to build Yukos into an international energy company.
"But the situation as it stands today compels me to give up
plans for further personal participation in developing the
YukosSibneft company," he said.
"As the leader of this company, I must do
my utmost to lead our working team out from under the attack
which has been directed against me and my partners."
Many Russians believe the case against Mr Khodorkovsky, who made his fortune through controversial privatisations in the 1990s, is politically motivated.
He has funded opposition groups, breaking what analysts say was a tacit agreement to stay out of politics in return for avoiding investigation of his financial affairs.
The crackdown on Yukos and its boss has stirred fears of a wider confrontation between the Kremlin and big business.
Last week, Russian prosecutors froze more than 40% of Yukos' shares, unnerving foreign investors and triggering a slump on the Russian stock market.
Analysts have said the Yukos affair ranks as the biggest political and economic crisis of President Vladimir Putin's three-year period in office.
Investors are reassessing Mr Putin's credentials as manager of one of the world's largest emerging economies, while political commentators are interpreting the action as a drive by Kremlin hardliners to suppress political dissent.
Mr Khodorkovsky's arrest came ahead of parliamentary elections in December, and a presidential election in March.
A BBC correspondent said his decision to step down would be seen by liberals and reformists in Russia as a victory for conservative forces in the Kremlin, who appear to be flexing their muscles ahead of the elections.
But Mr Khodorkovsky's departure was seen as positive for Yukos, lifting the company's share price by 13% to $12.65 on Russia's RTS stock exchange.
Mr Khodorkovsky, currently awaiting trial in Moscow's Matrosskaya Tishina jail, said he planned to concentrate on charity work for the Open Russia Foundation - a body he founded to promote a transparent and democratic society in Russia.
"Wherever I work, I will dedicate all my energy to my country, Russia, in whose great future I firmly believe," he said.
Separately, a lawyer acting for Mr Khodorkovsky accused Russian prosecutors of flouting the rule of law.
"We are indeed talking about a cancer, a cancer that is growing on the Russia body politic," said Robert Amsterdam of Toronto-based law firm Amsterdam & Peroff.
"That cancer is authoritarianism. That cancer is a complete and total disregard for the rule of law."
Mr Amsterdam was speaking during a visit to Brussels aimed at raising awareness of Mr Khodorkovsky's plight among EU officials ahead of an EU-Russia summit on Thursday.