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In February, Kyiv said the leader of the Opposition Platform – For Life party had escaped from house arrest.
Last year authorities opened a treason case against Mr Medvedchuk, who denies any wrongdoing.
"You can be a pro-Russian politician and work for the aggressor state for years," the security services said in an online post.
"You may have been hiding from justice lately. You can even wear a Ukrainian military uniform for camouflage.
"But will it help you escape punishment? Not at all. Shackles are waiting for you and the same goes for traitors to Ukraine like you."
The post quoted Ivan Bakanov, head of the secret services, as saying his operatives had "conducted a lightning-fast and dangerous multi-level special operation" to arrest Mr Medvedchuk.
A spokesman for Mr Medvedchuk was not immediately available for comment.