Russian opposition leader Alexei Navalny has been detained in Moscow, just two months after his release from prison for organizing protests against the government of President Vladimir Putin.

A spokeswoman for Navalny said Saturday the reason for the detention is unclear that and he was being held at the Danilovsky police station.

Navalny had posted on his blog Saturday that protests against the Putin government would take place September 9 in Moscow and “almost a hundred other cities.” The protests were against Putin’s pension reform plans.

September 9 is also the date of Moscow’s mayoral election.

Navalny’s spokeswoman, Kira Yarmysh, told radio station Ekho Moskvy that Navalny’s seizure by authorities is “probably linked” to the protest plans.

Navalny has faced a string of charges for his opposition activism. In March, he was barred from running in the country’s presidential election because of his criminal record.