Russian authorities stepped up pressure on dissent ahead of parliamentary elections, arresting an opposition activist and raiding the homes of several others.

Andrei Pivovarov, the leader of the Open Russia movement which disbanded last week, was pulled from a plane at St. Petersburg airport on Monday evening and was due to be taken to Krasnodar in southern Russia on Tuesday, as part of a criminal investigation against him.

Also on Tuesday, police raided the country house of opposition politician Dmitry Gudkov, a former lawmaker who aspired to run for parliament in September. At least two of his associates also searched their homes.

These measures are part of a multi-pronged crackdown on the opposition that is widely seen as part of the authorities’ efforts to prevent any opposition group from challenging the main Kremlin-backed United Russia party in the parliamentary elections of September. The popularity of United Russia has waned amid the country’s economic downturn.

Last week, Pivovarov announced that Open Russia was shutting down to protect its members from prosecution after authorities called it “junk.” The government has already banned more than 30 groups using a 2015 law that made membership in “undesirable” organizations a criminal offense. Another bill that is currently making its way through Parliament strengthens sanctions against their members.

Open Russia was funded by magnate Mikhail Khodorkovsky who moved to London after spending 10 years in prison in Russia on charges widely seen as political revenge for challenging President Vladimir Putin’s regime.

In March, police briefly arrested around 200 participants in a forum of city council members that Open Russia helped organize.

Putin’s most determined political enemy, Alexei Navalny, was arrested in January on his return from Germany, where he had spent five months recovering from nerve poisoning he attributes to the Kremlin – accusations that Russian officials reject. He was sentenced to two and a half years in prison in February for violating the terms of a suspended sentence stemming from a 2014 embezzlement conviction which he denounced as politically motivated.

With Navalny in prison, prosecutors have asked a Moscow court to designate his Anti-Corruption Foundation and network of regional offices as extremist groups. Meanwhile, a bill approved by Russia’s lower house of parliament bans members, donors and supporters of extremist groups from running for public office – a move that would bar Navalny’s associates from running for parliament in September.