Leonid Volkov, a senior aide to Navalny, tweeted on May 21 that he was detained by Moscow police after they stopped his car and asked him "strange questions."

Volkov said he was charged with repeated violations of the regulations for holding public events in connection with a September rally he helped organize against the government's decision to raise the retirement age.

Volkov is a project manager at Navalny's Anticorruption Foundation and led his campaign in the March 2018 presidential election.

Navalny was barred from taking part in the election because of past convictions that he and his supporters say were fabricated to keep him out of electoral politics.

On September 9, Navalny and his supporters organized mass rallies against the government's decision to raise the retirement age.

Police detained more than 1,000 demonstrators across the country.