The embattled head of A Just Russia's Astrakhan branch became the first prominent victim of tough new protest rules signed into law earlier this month.
An Astrakhan court ruled Friday that Oleg Shein should pay 20,000 rubles ($600) for taking part in an unsanctioned rally.
New rules enacted June 8 stipulate maximum penalties of 300,000 rubles ($9,275) for participants and 1 million rubles for organizers of unauthorized or illegal protests.
Police arrested Shein on June 12 after the opposition leader had marched with supporters along the city's main embankment and joined up with a sanctioned Liberal Democratic Party rally, RIA-Novosti reported.
Shein said in his defense that he was simply celebrating the Russia Day public holiday with like-minded people.
"It seems that I will be the first to be punished under the new law banning rallies. My guilt lay in the fact that when I walked along the Volga embankment, several women shouted 'Oleg Shein is our mayor,'" Shein wrote on his LiveJournal blog.
Shein has been in the media spotlight since going on hunger strike for 40 days in March after losing out to a United Russia candidate in disputed mayoral elections.
At the time, opposition leaders from Moscow rallied behind Shein, visiting him in Astrakhan and protesting the vote.
Central Elections Commission head Vladimir Churov later admitted there were numerous violations in the mayoral contest, but the election result stood.