The members of the neo-leftist Red Youth Vanguard, including its leader, Sergei Udaltsov, were demonstrating against the December 6 shooting by Greek police of a 15-year-old boy. Alexandros Grigoropoulos's death has sparked a week of youth rioting across Greece.
"The Red Youth Vanguard members tried to hold an unauthorized picket of the Greek Embassy in Moscow, and after repeated requests to disperse they were arrested by police," the spokesman said.
The eight were taken to a local police station for processing.
In cities throughout Greece on Saturday peaceful rallies were held in memory of Grigoropoulos. One policeman has been charged with his murder, and a second as an accomplice.