(Reuters) - Russia on Wednesday told Ukraine to "watch out" after its former Soviet neighbour captured pro-Kremlin politician Viktor Medvedchuk, turning down Kyiv's offer of a swap with a warning that those holding him might soon be detained themselves.

       Medvedchuk, one of President Vladimir Putin's close allies in Ukraine, was shown handcuffed and wearing the uniform of a Ukrainian soldier on Tuesday in a picture tweeted by President Volodymyr Zelenskiy.

       Zelenskiy proposed swapping Medvedchuk, while Ukraine's SBU domestic security service cast him as a traitor whose future would be in shackles.

       "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.

       "These people should watch out and lock the doors well at night to make sure they do not become the people who are going to be exchanged themselves," said Medvedev, a close Putin ally who served as Russian president from 2008 to 2012.

       In February, Ukraine said Medvedchuk, leader of the Opposition Platform - For Life party, escaped from house arrest. Authorities last year opened a treason case against Medvedchuk, who denies wrongdoing.

       The Kremlin said the court case against Medvedchuk was politically motivated and denied that he had any communication back channel to the Russian leadership.

       "He had no backstage relationship with Russia," Kremlin spokesperson Dmitry Peskov told reporters on Wednesday.

       "We will follow the fate of Viktor Medvedchuk - and we also call on European politicians to do the same as they are always so concerned about freedom of speech," Peskov said.

       Ukraine's SBU warned all "pro-Russian traitors and agents of the Russian secret services" that they would be brought to justice "for all the crimes of the present day".

       Thousands of people in Ukraine have been killed and nearly 10 million displaced after Russia sent troops into Ukraine on Feb. 24, raising fears of a wider confrontation between the United States and Russia - the world's two biggest nuclear powers.

       Putin says what he calls the "special military operation" in Ukraine is necessary because the United States was using Ukraine to threaten Russia and Moscow had to act to defend Russian-speaking people in Ukraine against persecution.

       Ukraine says it is fighting against an imperial-style land grab and dismisses Putin's claims of genocide as nonsense.

       Asked about the Ukrainian proposal to swap Medvedchuk for Ukrainians being held by Russia, Peskov said Medvedchuk was a Ukrainian citizen and a foreign politician.

       (Reporting by Reuters; Editing by Conor Humphries)