Russian opposition leader Alexei Navalny has been detained in Moscow, his spokeswoman Kira Yarmysh wrote on Twitter on Saturday.

Yarmysh said Navalny had been taken to a local police station and that the reason for his detention was unclear.

Navalny, who has been detained and jailed several times for organizing anti-Kremlin protests, was barred from running in Russia’s presidential election earlier this year on what he says was a false pretext.

The 42-year-old was last sentenced to 30 days in prison in May for his role in organizing nationwide protests against President Vladimir Putin.

He had called for demonstrations in more than 90 towns and cities under the slogan “Putin is not our tsar” to protest what he says is the Russian president’s autocratic rule.

Putin, 65, won re-election overwhelmingly in March, extending his grip over Russia for six more years.