The director of Russian opposition politician Aleksei Navalny's Anticorruption Foundation (FBK), Roman Rubanov, was briefly detained and told to return to a law enforcement agency for questioning on October 13.
Rubanov wrote on Twitter that he was detained by court officers on October 10 near his apartment building in Moscow.
He said the officers told him he was detained in connection with investigations into FBK's refusal to remove Internet posts it has been ordered to take down following a defamation suit filed by Kremlin-connected tycoon Alisher Usmanov.
Usmanov filed the lawsuit in April after Navalny released a video, based on an FBK investigation, alleging that Prime Minister Dmitry Medvedev has used a network of purported charitable foundations to control wealth in Russia and abroad.
In May, a Moscow court ordered Navalny to remove the publications that mention Usmanov and to publish a retraction.
Rubanov told the news site Meduza that his detention was an attempt to frighten him and Navalny.
With reporting by Meduza