The Zmiana (Change) party claimed today's move against Mateusz Piskorski and other police actions against the organisation were politically motivated and accused Poland's rightwing government of intimidation.
Prosecutors did not disclose the specific allegations against him but Polish media reported that Piskorski, who founded Zmiana in 2015, was suspected of spying for Russia, and also possibly China.
He controversially visited Crimea to show his support for the Russian annexation of the peninsula from Ukraine in 2014.
Rekas blasted the actions as "an attempt to intimidate those whose views on foreign, domestic and socio-economic policy differ from those of the government".
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Elected in October, Poland's nationalist Law and Justice (PiS) government is wary of Russia and staunchly opposed its takeover of Crimea.
Earlier this month, Polish prosecutors charged a Polish-Russian dual citizen with spying for Moscow, while in January, fellow EU and NATO member Estonia jailed ethnic Russian cigarette smugglers for spying for Russian secret services.