Russia's wealthiest man has been called to a prosecutor's office while his major ally faces charges of theft of state property in connection with a 1990s privatisation deal.
YUKOS boss Mr Mikhail Khodorkovsky, chief executive of oil company YUKOS, is reputed to be worth $11 billion. He appeared as scheduled at a prosecutor's office in the case concerning the 1994 privatisation of a fertiliser company.
His accused ally, Mr Platon Lebedev, head of a subsidiary controlling a large chunk of YUKOS shares, has been questioned since being detained on Wednesday.
Mr Lebedev's spokesman said he had been charged with theft of state property and contempt of court, though officials close to him said they had not been provided with full explanation.
Business figures have said the legal action against Mr Lebedev had been initiated by Kremlin hawks keen on clipping the wings of "oligarchs", the super-wealthy elite, before December parliamentary elections.
That contest is to be followed by a March 2004 presidential election in which Kremlin leader Mr Vladimir Putin is widely expected to seek a second term.
Mr Khodorkovsky has publicly supported the liberal opposition against Mr Putin's allies in parliament.
The alleged offence took place in 1994, the General Prosecutor's office said - at the peak of Russia's post-Soviet mass sales of state assets which enabled a small group of entrepreneurs to acquire state-owned companies cheaply.
Mr Lebedev, himself worth $1.3 billion according to Forbesmagazine, was arrested a day after another wealthy "oligarch", Mr Roman Abramovich caused a stir in Britain after it was announced he had acquired prestigious English soccer club Chelsea.