Police detained Russian opposition politician Alexei Navalny on Thursday during a raid on the headquarters of his Anti-Corruption Foundation (FBK) in Moscow, his spokeswoman said on social media.
"Alexei was forcibly detained and taken away. He did not resist. Lawyers are still at the FBK, and there's a search underway," Kira Yarmysh, Navalny's spokeswoman, wrote on Twitter.
CCTV footage of the raid showed police using power tools to saw through the front door. A group of men, some of them masked, could then be seen searching the office before one of them covered a CCTV camera up with tape.
There was no immediate comment from the police.
The raid occurred a day after Navalny said that the forcible military conscription of one of his allies to a remote air base in the Arctic amounted to kidnapping and illegal imprisonment.
Allies said Navalny, who was barred from running in a presidential election against Vladimir Putin last year, had been due to present his own show, which is critical of the authorities, on his online TV channel later on Thursday.
It was not immediately clear why Navalny had been detained, but his foundation, which specialises in publishing exposes on the corruption of state officials, is the subject of a criminal investigation into alleged money laundering.
Investigators opened the money laundering case in August after Navalny called for people to demonstrate in central Moscow over the exclusion of opposition candidates from a local election.
Those protests grew into Moscow's biggest sustained protest movement in years before fizzling out.
The Justice Ministry in October formally labelled Navalny's anti-corruption group a "foreign agent", meaning it can be subjected to spot checks and face bureaucratic scrutiny.
Navalny has called that move and others part of a coordinated and trumped up campaign to stifle the anti-Kremlin opposition's activities.