Security officers in Moscow have arrested Alexei Navalny, the most prominent foe of President Vladimir Putin and the governing United Russia party.
Mr Navalny has been jailed repeatedly in recent years for organising or participating in unsanctioned protests.
His Foundation for Fighting Corruption organisation has produced reports alleging corruption by top figures including the prime minister Dmitry Medvedev.
Mr Navalny was detained when officers forced their way into his organisation's office on Thursday, his spokeswoman said.
An attorney with Mr Navalny's foundation said the officers were from the federal court bailiffs' service and that they claimed to be searching for evidence connected to a case against the organisation's director Ivan Zhdanov.
A criminal case was opened against Mr Zhdanov earlier this year for failing to follow an order to remove a video version of a report claiming corruption by Mr Medvedev.
The federal court bailiffs' service later said Mr Navalny had not been detained.
