An ally of Russian opposition leader Alexei Navalny has been sentenced to 15 days in jail on the day he was to be released.

A Moscow court on Monday found Leonid Volkov guilty of violating rules for holding a rally last year.

Navalny is the most prominent foe of Russian President Vladimir Putin. Volkov, who is often described as Navalny's right hand, helped organize rallies in several Russian cities last September to protest the government's decision to raise the pension age.

Volkov was sentenced to 20 days in jail last month on the same charge, related to a rally in Moscow. He was detained just as he was getting released Monday and charged with violations related to a rally in St. Petersburg.