Grigory Torbeyev, a member of the Left Front opposition organization, hit a police officer in the face with a burning flare as officers tried to disperse the Day of Wrath rally in Moscow's Tverksaya Square in November 2010, RIA Novosti reported Monday.
The man was arrested at the scene of the incident. The officer, Yevgeny Kovalev, was taken to a hospital and treated for eye injuries and a concussion.
Prosecutors initially asked that Torbeyev be sentenced to two years in a penal colony, but agreed to one year.
Torbeyev's lawyers expressed outrage at the verdict and said they plan an appeal.