A Russian court has banned organizations linked to imprisoned opposition leader Alexei Navalny, declaring them to be “extremist.” The move is seen as an effort to curb allies of the Kremlin’s loudest critic from running for parliamentary elections in September.

RFERL reports that a ruling by the Moscow City Court on June 9 immediately came into force, preventing anyone associated with Navalny’s Anti-Corruption Foundation and his network of regional allies across the country from seeking public office. The ruling also carries lengthy prison terms for activists who have worked with the now-prohibited organizations.

Prosecutors in Moscow claimed that “under the guise of liberal slogans,” organizations friendly to Navalny were “engaged in creating conditions for destabilizing the social and sociopolitical situation.”

Defence lawyer Yevgeny Smirnov argued that the prosecutors’ motion to ban the organizations was designed to bar Navalny’s supporters from running for office.

“This case has been linked to the law that bans all those who are connected with the FBK from getting elected,” Smirnov said. He said they would appeal the ruling, which was made in a closed-door court session on the grounds that classified materials would be discussed.

According to the RFERL, authorities have stepped up their prohibitions on dissent as opinion polls show decreasing support for President Vladimir Putin’s ruling United Russia party.

Navalny, a fierce critic of Putin, was arrested in January upon his return from Germany, where he spent five years recovering from poisoning caused by a nerve agent. Navalny has levelled accusations against the Kremlin for the poisoning.

Upon arrival in Russia, Navalny was arrested and later sentenced to 2 ½ years in prison for violating the terms of a suspended sentence from a 2014 conviction for embezzlement, considered to be politically motivated.