Vladimir Kara-Murza pleaded with Russians not to "give up" after the death of fellow Vladimir Putin critic Alexei Navalny as he appeared in court last week.
The opposition figure, a British-Russian citizen, is serving a 25-year sentence for treason.
He appeared in court on Thursday, 22 February, giving a speech to bring a complaint against Russia's Investigative Committee for not properly investigating what he believes were two poisoning attempts against him.
Mr Kara-Murza was found guilty of criticizing Russia's invasion of Ukraine and was jailed as part of an ongoing crackdown by Russian authorities against critics of the "military operation" and freedom of speech.