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Ukraine's security services on Tuesday said they had arrested pro-Moscow politician Viktor Medvedchuk, who is Russian President Vladimir Putin's closest and most influential ally in the country.
Ukrainian President Volodymyr Zelenskyy had earlier published a photo of a tired and handcuffed Mr Medvedchuk, who says Mr Putin is godfather to his daughter.
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."
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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.
Last month Mr Zelenskyy said the Opposition Platform – For Life, which is Ukraine's largest opposition movement, and other smaller political parties with ties to Russia had been suspended.
A spokesman for Mr Medvedchuk was not immediately available for comment.