MOSCOW (AP) — Russian authorities have ramped up the pressure on dissent ahead of a parliamentary election in September, arresting one opposition activist and raiding several others' homes.

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

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

The moves are part of a multi-pronged government crackdown on the opposition that is widely seen as part of the authorities' efforts to prevent any opposition groups from mounting a challenge to the main Kremlin-backed United Russia party in the Sept. 19 parliamentary election. United Russia's popularity has waned amid the country's economic slowdown.

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."

His lawyers said the charges against Pivovarov for assisting an “undesirable” organization were filed after he declared its closure.

Amnesty International, meanwhile, strongly condemned Pivovarov's arrrest 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."

Open Russia was financed by tycoon 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 rule.

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.