Moscow (CNN) It's been a heady summer for Russia's embattled political opposition. On August 10, as many as 50,000 people rallied in Moscow calling for fair local elections, the biggest protests seen in the capital since early 2012. Now opposition leaders are calling for a protest on August 31, hoping to build on the momentum of Moscow's summer of discontent.

So who are the protest leaders, and what is their goal?

Alexei Navalny is the most recognizable Russian opposition leader. In late July, ahead of unsanctioned protests, he was jailed amid a roundup of opposition figures ahead of unsanctioned demonstrations on July 27 and sentenced to 30 days for allegedly violating protest laws. While in custody, he was hospitalized with an "acute allergic reaction." His physician suspected poisoning by an unknown substance.

Navalny was released from jail on Friday, but while he was inside another activist has emerged as a leading opposition voice: Lyubov Sobol, a lawyer and activist with Navalny's Anti-Corruption Fund. Sobol recently ended a month-long hunger-strike after election officials refused to allow her onto the ballot in upcoming municipal elections; she was also detained and subsequently released ahead of an August 3 protest.

Alexei Navalny is arrested at a rally in January 2019.

Initially, the protests centered on those municipal elections, which are scheduled for September 8. Moscow's election commission has barred a number of independent and opposition candidates from running because they had failed to obtain a sufficient number of signatures to be allowed to run. Opposition activists say the authorities are using administrative measures to block true political competition.

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