Leonid Razvozzhayev, an aide to the A Just Russia parliamentary deputy, Ilya Ponomaryov, has been brought to Moscow from Kyiv after being apprehended in Ukraine, according to opposition activist Sergei Udaltsov.
"He is in Moscow and police want to arrest him," Udaltsov told the Interfax news agency.
Razvozzhayev had been sought by Russian investigators probing allegations of preparations for mass riots.
Ilya Ponomaryov, from the center-left opposition party A Just Russia, told Interfax on October 20 that Razvozzhayev was kidnapped when leaving the offices of the UN High Commissioner for Refugees (UNHCR) in Kyiv.
Interfax said police in Kyiv had no information on the subject.
Ponomaryov said he had been told of Razvozzhayev's kidnapping by colleagues in a left-wing Ukrainian political movement.
Another Russian opposition activist, Konstantin Lebedev, has been charged with preparing to organize mass riots, and a criminal probe has been launched against Left Front leader Sergei Udaltsov.
Based on reporting by AFP, Reuters, Interfax, and UNIAN