MOSCOW (AP) — Russian authorities are cracking down on dissent before a crucial parliamentary election in September, in what a leading Kremlin critic on Tuesday described as an attempt to sideline opponents.

Mikhail Khodorkovsky, a Russian tycoon who moved to London after spending a decade in prison in Russia on charges widely seen as political revenge for challenging President Vladimir Putin’s rule, said the latest moves against opposition activists reflected the authorities' concern about the waning popularity of the main Kremlin-directed party, United Russia.

Khodorkovsky told The Associated Press in an interview over Zoom that the upcoming election is a “theatrical performance, in which any candidates that the government isn't happy with will simply not be allowed to run.” He said that the authorities are increasing repression to stifle any critical voices before the Sept. 19 parliamentary election, including activists of the Open Russian movement that he financed.

Open Russia dissolved itself last week. Andrei Pivovarov, the organization's head, was pulled off a Warsaw-bound plane at St. Petersburg's airport just before takeoff late Monday. Pivovarov was taken on Tuesday to Krasnodar in southern Russia as part of a criminal probe targeting him.

Also Tuesday, police raided the country home of opposition politician Dmitry Gudkov and detained him. Gudkov, a former lawmaker, has aspired to run for Russia's parliament. At least two of his associates also had their homes searched.

Khodorkovsky said the authorities are taking preemptive action against the opposition, fearing the September election could trigger protests.

“The government is afraid of potential protests that could emerge if they cheat too flagrantly,” Khodorkovsky told the AP. “They are trying to sanitize the political environment before the election.”

He added that he would continue to support opposition candidates despite the official pressure.

Last week, Pivovarov announced that Open Russia was shutting down to protect its members from prosecution after Russian authorities designated it as “undesirable.” The government already has outlawed more than 30 groups using a 2015 law that made membership in “undesirable” organizations a criminal offense. Another bill now making its way through parliament increases the punishment for members of those groups.

In a letter from custody that his lawyers posted Tuesday on Pivovarov's Instagram account, he said “there is no cause for joy, but I don't feel despondent.”

He added: “There is a plan to put any people with a different view under arrest, but such people already are the majority."

The probe focuses on Pivovarov’s support for a candidate in local elections on behalf of an “undesirable” organization. His lawyers said the charges were filed after he declared Open Russia's closure.

Amnesty International, meanwhile, strongly condemned Pivovarov's arrest and demanded his release.

“This is an audacious move by the Kremlin in its continued use of the law on ‘undesirable’ organizations to target and shut down critics," said Natalia Zviagina, the director of the group's Moscow office. “The Russian authorities must end reprisals against their political opponents and other critical voices in the country."

In March, police briefly detained about 200 participants of a forum of municipal council members that Open Russia helped organize.

Putin's most determined political foe, Alexei Navalny, was arrested in January upon his return from Germany, where he had spent five months recovering from a nerve agent poisoning that he blames on the Kremlin — accusations that Russian officials reject. He was handed a 2 1/2-year prison sentence in February for violating terms of a suspended sentence stemming from a 2014 embezzlement conviction that he denounced as politically driven.

With Navalny in prison, prosecutors have asked a Moscow court to designate his Foundation for Fighting Corruption and his network of regional offices as extremist groups. In a parallel move, a bill approved by the lower house of the Russian parliament bars members, donors and supporters of extremist groups from seeking public office — a measure that would keep Navalny's associates from running for parliament in September.

Khodorkovsky argued that the parliamentary election is important for Putin to cement his rule ahead of the 2024 Russian presidential election.

The 68-year-old Putin has been in power for more than two decades — longer than any other Kremlin leader since Soviet dictator Josef Stalin/ He pushed through constitutional changes last year that enable him to run again in 2024, when his current six-year term ends. The changes would allow him to potentially hold onto power until 2036.