Alexei Navalny is suing the Kremlin over claims that the Russian opposition leader has been working hand in hand with the US intelligence.
Mr Navalny, who spent weeks in a coma after a nerve agent poisoning, said on Tuesday that he has filed a lawsuit against Kremlin spokesman Dmitry Peskov over his remarks about Mr Navalny's alleged ties to the CIA.
"I don't typically sue propagandists... but the important thing here is that Peskov is not merely a person whose job is to spread lies. He is a deputy chief of presidential staff," Mr Navalny said in an Instagram post, calling Mr Peskov a "mustachioed liar."
Mr Peskov on Tuesday refused to comment on the lawsuit against him.
The 44-year-old opposition leader who ran an impressive campaign across Russia before he was barred from running for president in 2018 fell suddenly ill on a plane from Siberia to Moscow in August and spent three weeks in a coma before recovering.
Several European laboratories independently confirmed that Mr Navalny was poisoned with the Soviet-era nerve agent Novichok which was used in the 2018 attack on former Soviet spy Sergei Skripal and his daughter.
Mr Navalny has pointed the finger at President Vladimir Putin as the only person who could have ordered the Novichok attack.
The European Union last month imposed sanctions on six top Russian officials and a chemical research centre over the Navalny poisoning.
The Kremlin has denied any involvement in the attack and vowed to slap European officials with visa bans in retaliation.
The original claim about Mr Navalny's alleged ties to the US intelligence was made by Russian parliament speaker Vyacheslav Volodin who accused Mr Navalny of being a CIA spy.
Mr Peskov, the Kremlin spokesman, mentioned the allegations in September, claiming that Mr Navanly works with "CIA specialists" who "put in his mouth everything that he says."
President Putin's most formidable foe, Mr Navalny was discharged from a Berlin hospital in September and remains in Germany to continue his treatment. He has vowed to return to Russia as soon as he has made a full recovery.