Criminal charges are being filed against four more demonstrators who allegedly incited mass riots and violence at the May 6 opposition rally, which ended in clashes with police and hundreds of arrests.
The Investigative Committee on Monday identified the defendants as Stepan Zimin, Denis Lutskevich, Rikhard Sobolev and Oleg Arkhipenkov.
Another four demonstrators were recently hit with the same charges and face lengthy prison terms.
Maria Baronova, an aide to State Duma Deputy Ilya Ponomaryov, was also summoned so that related charges could be filed against her "in the coming days," the committee said.
Several police officers were injured and more than 400 people detained at the May 6 rally on Bolotnaya Ploshchad, across the Moscow River opposite the Kremlin.
As part of an investigation into violence at that protest, police recently raided the homes of several opposition activists and called them in for questioning at the same time that another major rally was scheduled.
During the raids, authorities confiscated more than $1.7 million in cash from the apartment of TV personality-turned-opposition activist Ksenia Sobchak.
The Investigative Committee Monday that officials were scrutinizing Sobchak's income tax declaration for last year, which she filed on May 3. The results should be reported within three months.
Izvestia reported Monday that investigators are considering charging Sobchak with tax evasion. Sobchak denounced such charges as "slander" and "fabrication."