Navalny, President Vladimir Putin's most prominent critic, was jailed this month for almost three years for parole violations he said were trumped up. The West has condemned the case and is discussing possible sanctions on Russia.
In the slander case, Navalny stands accused of defaming a World War Two veteran who took part in a promotional video backing constitutional reforms last year that let Putin run for two more terms in the Kremlin after 2024 if he wants.
Navalny described the people in the video as traitors and corrupt lackeys. He accuses authorities of using the slander charges to smear his reputation.