A spokesman for Alexander Lebedev says the Russian banking and media tycoon is being held inside the Moscow headquarters of a bank he owns while masked police rifle through company files.
Artyom Artyomov told The Associated Press that up to 30 armed Moscow city policemen surrounded Lebedev's National Reserve Bank on Tuesday and entered the premises.
He said he knew of no reason for the raid. A city police spokesman refused to comment.
Lebedev recently acquired Britain's Independent newspaper, owns the London Evening Standard, has a stake in Russia's Novaya Gazeta newspaper and is part-owner of national carrier Aeroflot.
Raiding company property and seizing computers and documents is common practice among Russia's law-enforcement agencies.