Aleksei Navalny, a prominent leader of Russia's opposition movement against President Vladimir Putin's rule, has called for a nationwide protest in early September.
Navalny was speaking on May 24 after his release from jail after serving 15 days on charges of resisting police in connection with a May 9 protest.
Navalny also called on supporters to join a protest rally on June 12.
Navalny has parlayed his status as an anticorruption blogger into star status among those disgruntled with Kremlin maneuvering.
Fellow opposition activist Sergei Udaltsov of the Left Front group was also released after serving 15 days.
He immediately headed for a gathering of opposition activists at the Arbat in Moscow.
Udaltsov joined activists near the monument to well-known Soviet-era musician Bulat Okudzhava, urging supporters not to give up their protests against the return of Putin to the presidency and the government.
Udaltsov told ITAR-TASS he would attend a public hearing on a new bill to stiffen penalties for holding or participating in unsanctioned demonstrations later in the day and would then spend time planning the next protest scheduled for June 12.
Russia's opposition has staged a series of unprecedented mass protests against alleged electoral fraud and the 12-year rule of Putin, who this month began a third presidential term.
Putin has backed a measure currently under consideration in the State Duma for a 200-fold increase in fines for those taking part in unsanctioned protests.
With additional reporting by AFP, Interfax, and ITAR-TASS