Civic Irkutsk leader Timofei Kirichenko said two assailants dressed in camouflage uniforms and military boots attacked him Wednesday evening outside the entrance to his apartment building.
"They threw me down to the ground and started to kick me, after which they fled in a white foreign-make car," he told Interfax.
He said he sustained a concussion as a result of the beating. Local police are investigating the report.
Kirichenko's activist group organized For Fair Elections demonstrations in Irkutsk in recent months protesting falsifications in December's State Duma elections, and has recruited observers for Sunday's presidential vote.