The Babushkinsky Court on Tuesday found anti-corruption leader Alexei Navalny guilty of slandering a municipal lawmaker and ordered him to pay a fine of $8,400 (6,079 euros).
Navalny was sued by municipal deputy Alexei Lisovenko for calling him "a lawmaker-drug addict" on the social networking site, Twitter.
The prominent critic of Russian President Vladimir Putin is also facing embezzlement charges in a separate case opening on Thursday. Navalny has called the charges politically motivated.
Pending the trial he has been under house arrest for two months and has been barred from using the Internet but his wife and associates say they have been posting on his behalf.
Navalny rose to prominence as an anti-corruption blogger during mass protests in the winter of 2011-2012 against Putin's return to the Kremlin for a third term. Last September he took nearly a third of the votes in Moscow's mayoral election. However, a separate suspended sentence on a theft conviction will prevent him from being eligible to run in the next presidential election in 2018.
hc/jlw (AFP, AP)