Sergei Makhnatkin was sentenced on June 9 to 2 1/2 years in jail for allegedly breaking a policeman's nose during a December 31 protest in Moscow that was forcibly dispersed by police.
Opposition leaders and Makhnatkin himself have said repeatedly that he was not participating in the protest, but only stopped to defend an elderly woman who was being harassed by police.
Sergei Davidis, a Solidarity movement leader, told RFE/RL that the protest on July 13 was held to bring attention to the plight of political prisoners in Russia.
The protest follows a demonstration by more than 100 people for Makhnatkin in St. Petersburg on July 12.
Representatives of several oppositions parties including Defense, Yabloko, and the Partnership of Determined Russian Citizens gathered around a giant banner reading "Freedom for Sergei Makhnatkin."
Olga Kurnosova, leader of the opposition United Civil Front, told RFE/RL that what happened to Makhnatkin was related to what happened to two organizers of the art exhibition "Forbidden Art," who were recently convicted, and could happen to any Russian citizen.
"In Russia, courts think that lawyers and police officers never lie," she said. "Even if the truth was on tape they wouldn't watch it in the courtroom."
Protesters in St. Petersburg said that next week they will start gathering signatures and sending out mass e-mails in support of Makhnatkin.