Ukrainian President Volodymyr Zelensky on Wednesday proposed swapping senior pro-Russian politician Viktor Medvedchuk for male and female prisoners of war being held by Moscow's forces.
In an early morning address, he said it was "important for our security forces and military forces to consider such a possibility."
Ukraine's security services on Tuesday said they had arrested the pro-Russian politician, who is President Vladimir Putin's closest and most influential ally in Ukraine.
President Volodymyr Zelensky had earlier published a photo of a tired-looking and handcuffed Medvedchuk, who says Putin is godfather to his daughter.
"Those freaks who call themselves the Ukrainian authorities say that they want to beat testimony out of Viktor Medvedchuk, 'quickly and fairly', convict him, and then exchange him for prisoners," Dmitry Medvedev, deputy chairman of Russia's Security Council, said.
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 the same goes for traitors to Ukraine like you."
Ти можеш бути проросійським політиком і роками працювати на державу-агресора. Ти можеш ховатися від правосуддя останнім часом. Можеш навіть одягнути українську військову форму для маскування… Але чи допоможе тобі це втекти від покарання? Аж ніяк! Кайдани чекають на тебе. 1/4 pic.twitter.com/MmCNLVXiAJ
— СБ України (@ServiceSsu) April 12, 2022
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.
Medvedchuk, a Russian-born, lawyer and wealthy businessman who served in political roles since the 1990s, he has often been photographed with Putin.
Medvedchuk served as chief of staff of Ukrainian president Leonid Kuchma from 2002-2005. He was an opponent of Kuchma's pro-Western successor Viktor Yushchenko, and a close associate of Viktor Yanukovych, Ukraine's last pro-Russian president, who fled to Russia after being ousted in a popular uprising in 2014.
During the conflict with Russia in eastern Ukraine in 2014, he played a role in peace negotiations, including serving as a member of Ukraine's negotiating team for the exchange of prisoners.
During the conflict with Russia in eastern Ukraine in 2014, he played a role in peace negotiations, including serving as a member of Ukraine's negotiating team for the exchange of prisoners.
According to Ukrainian authorities, Medvedchuk was involved in the illegal sale of coal from separatist-controlled territories in the east of Ukraine. Last year, Ukraine nationalised an oil products pipeline owned by Medvedchuk and closed several television channels which the media associated with him.