Russian police have raided the home of opposition activist Lyubov Sobol early this morning and took her in for questioning, Kremlin critic Alexei Navalny and his supporters said.
Mr Navalny's supporters said they thought the police action was a response to her trying to knock at the door of an alleged FSB security agent in Moscow who Mr Navalny says took part in a botched plot to poison him in August.
The FSB has dismissed Mr Navalny's allegations as a provocation.
Ivan Zhdanov, head of Navalny's Anti-Corruption Fund, wrote on Twitter that investigators had launched an investigation into trespassing "with the use of violence or a threat to use it".
Mr Navalny, one of President Vladimir Putin's most outspoken critics, was airlifted to Germany for medical treatment in August after collapsing on a plane in Russia.
Germany has said he was poisoned with a Soviet-style Novichok nerve agent in an attempt to murder him, an assertion many Western nations accept.
The Kremlin has repeatedly rejected any suggestion that Russia tried to kill Mr Navalny.
Mr Navalny on Monday released a recording of him speaking by phone at length to a man he described as a state security agent who told him, among other things, that the poison had been placed in his underpants.
Mr Navalny said he had posed as an aide to a senior Russian security official to gain the man's confidence.
Russia's FSB security service dismissed the recording as a fake.