The party led by jailed former Prime Minister Yulia Tymoshenko claims that President Viktor Yanukovych sanctioned the abduction of Leonid Razvozzhayev in order to secure the Kremlin's backing for Sunday's parliamentary election in Ukraine.
"The special services of a foreign state are operating on Ukraine's territory," said Tymoshenko ally Arseniy Yatsenyuk.
Yanukovych, who visited Moscow earlier this week, has been courting the Kremlin to seek a rebate on Russian natural gas supplies. A lower gas price is essential to keep the struggling Ukrainian economy afloat and would secure public support for Yanukovych and his party.
Razvozzhayev, a 39-year-old Russian opposition activist, had come to Kiev last week to seek political asylum in Ukraine after Russian investigators accused him of being involved in plotting anti-government riots.
Razvozzhayev told Russian human rights defenders who visited him in a Moscow jail that he was kidnapped on a Kiev street by masked men who smuggled him into Russia. He said they kept him in handcuffs and leg chains in a basement for three days, denying him food, water, sleep or use of a toilet. He also said they threatened to kill his children until he "confessed" to plotting riots.
Russian authorities say Razvozzhayev turned himself in and confessed voluntarily.
Razvozzhayev's lawyer, Mark Feigin, said on Ekho Moskvy radio Thursday that his client has formally disavowed his confessions and filed a complaint over his abduction to Russia's main investigative agency.
Ukraine's Foreign Ministry and the national security agency have declined to comment on the matter. But Interior Ministry spokesman Volodymyr Polishchuk on Thursday allowed for the possibility that Russian security services could have been involved in Razvozzhayev's abduction.
"How did it happen? Did they ask him to (go) of his own free will? Or did they just force him? I don't know," Polishchuk told The Associated Press.
Either way, Polishchuk said, what happened to Razvozzhayev did not constitute a crime because he is "alive and well" and the Interior Ministry will not press charges.
"This is not the province of the police. The police deal with criminal actions," he said.
Yatsenyuk vowed that the opposition would press for a parliamentary probe into Razvozzhayev's case.
Human Rights Watch, meanwhile, has urged Ukraine to thoroughly investigate Razvozzhayev's abduction.
"For an asylum seeker to simply vanish while lodging his asylum claims and then reappear in the country he fled is profoundly shocking," said Hugh Williamson, Europe and Central Asia director at Human Rights Watch. "There needs to be a serious investigation to determine whether any Ukrainian officials were involved and to hold accountable any who played a role."
The Office of the U.N. High Commissioner for Refugees also voiced concern about Razvozzhayev's fate, emphasizing Thursday that he was protected by international refugee law as an asylum-seeker.
The agency said it was seeking Russia's clarification of Razvozzhayev's condition and status and warned Ukraine that it must make sure that asylum seekers aren't exposed to personal risks.
The Russian Foreign Ministry responded angrily after the U.S. Embassy in Moscow voiced concern about Razvozzhayev's case, rejecting the U.S. statement as "hasty" and "improper" and noting claims that the CIA had tortured some detainees.
Razvozzhayev's case was reminiscent of the disappearance last year of a Palestinian engineer from a Ukrainian train. He surfaced in an Israeli prison several days later to face terrorism charges. Kiev has still failed to explain how that happened.