Putin's 'man in Ukraine' Viktor Medvedchuk has just been captured
President Volodymyr Zelensky had earlier published a photo of a tired-looking and handcuffed Medvedchuk, who says Putin is godfather to his daughter.
In February, Kyiv said Medvedchuk, the leader of the Opposition Platform - For Life party, had escaped from house arrest. Last year authorities opened a treason case against Medvedchuk, who denies wrongdoing.
"You can be a pro-Russian politician and work for the aggressor state for years. You may have been hiding from justice lately. You can even wear a Ukrainian military uniform for camouflage," the security services said in an online post.
"But will it help you escape punishment? Not at all! Shackles are waiting for you and same goes for traitors to Ukraine like you."
The post cited Ivan Bakanov, head of the secret services, as saying his operatives had "conducted a lightning-fast and dangerous multi-level special operation" to arrest Medvedchuk but did not give details.
Last month Zelenskiy said the Opposition Platform - For Life, which is Ukraine's largest opposition movement, and several other smaller political parties with ties to Russia had been suspended.
A spokesperson for Medvedchuk was not immediately available for comment.