Russian prosecutors have questioned and released the country's richest man after charging his main business ally with theft of state property.
Platon Lebedev's arrest has taken Russia by surprise
Mikhail Khodorkovsky, the chief executive of oil giant Yukos, was summoned to the prosecutor's office on Friday morning for questioning in connection with the 1994 privatisation of a fertiliser company.
Platon Lebedev, also one of Russia's richest men and a major shareholder in Yukos, was earlier charged with theft of state property in connection with the privatisation.
Mr Lebedev is accused of embezzling 20% of state-owned fertiliser company Apatit in 1994, making a personal profit of $283m (£169m).
A former deputy chief executive of Yukos, Leonid Nevzlin, was also called in by prosecutors for questioning on Friday.
Some analysts have interpreted the legal action against Mr Lebedev as a warning to Russia's business elite - the so-called oligarchy - against closer involvement in politics ahead of December's parliamentary elections.
Mr Lebedev and Mr Khodorkovsky are close business allies and two of Russia's most prominent oligarchs - the wealthy elite who made billions from the chaotic sell-off of state firms in the years following the collapse of the Soviet regime.
Roman Abramovich, the oil tycoon who is buying the UK's Chelsea football club, is another leading member of the Russian oligarchy.
Yukos is awaiting regulatory approval for a multi-billion dollar merger with Mr Abramovich's oil firm, Sibneft.
Mr Lebedev, who is worth an estimated $1.3bn, is chairman of Menatep, the finance group which controls 61% of Yukos.
Mr Khodorkovsky has supported liberal opposition groups
Mr Khodorkovsky described Mr Lebedev's arrest as illegal, and promised to protest officially.
Lawyers were not allowed to be present when the charges were read out on Thursday.
"I do not plan to leave Russia in the near future. I should have travelled to London today, but I decided to stay," Mr Khodorkovsky said.
"The only place I might travel is to Tomsk, for a meeting of the Yukos executive board," he added.
He also stressed that the merger with Sibneft would go ahead as planned.
The combined company would be the largest oil firm in Russia, controlling a third of the country's oil output, and with a market value of about $35bn.
Supporting the opposition
Russia's president, Vladimir Putin, has demonstrated his willingness to curb the country's oligarchs since his first day in office.
Roman Abramovich's firm plans to merge with Yukos
The move is widely supported in Russia, where the majority of the people regard the oligarchs as crooks.
Mr Putin's first victims were the media moguls Vladimir Gusinsky and Boris Berezovsky.
The arrest of Mr Lebedev was interpreted by many political analysts as a warning to his partner Mr Khodorkovsky against backing opposition groups in the run-up to elections.
Mr Khodorkovsky has publicly supported - and is reported to have financed - the two leading liberal parties, Yabloko and SPS, which will compete with pro-Kremlin parties in December's parliamentary elections.