Russia's Investigative Committee says Leonid Razvozzhayev, an aide to a State Duma deputy, has been officially charged with plotting mass riots.
The charges filed on October 23 are in connection with an opposition protest in Moscow in May.
Investigators say Razvozzhayev turned himself in to the authorities and confessed on October 22 that a Georgian official had financed the demonstration.
But Razvozzhayev's lawyer, Violetta Volkova, said that her client was kidnapped in Kyiv last week.
She says he was brought to Moscow against his will and suspects his confession is the result of torture.
Volkova says she's been denied access to Razvozzhayev.
Ukrainian opposition members are demanding explanations from the government regarding the alleged abduction.
The Ukrainian Foreign Ministry promised to comment on the situation in due time.
Based on reporting by Interfax, ITAR-TASS, and UNIAN