MOSCOW — Russian police on Tuesday conducted nationwide raids on the offices of opposition politician Alexei Navalny's anti-corruption foundation, part of a criminal investigation that Kremlin critics say is designed to thwart his activities.

Searches were conducted in relation to investigations taking place in 30 different regions, including in Moscow, into suspected money laundering, Russia's Investigative Committee, the state body that investigates major crimes, said.

Investigators opened the money laundering case in August after Navalny called for people to demonstrate in central Moscow over the exclusion of opposition candidates from a local election that took place last month.

Russian opposition leader Alexei Navalny speaks during a rally to support political prisoners in Moscow, Russia, Sunday, Sept. 29, 2019. Dmitri Lovetsky / AP file

Those protests grew into Moscow's biggest sustained protest movement in years before appearing to fizzle out.

The Justice Ministry earlier this month formally labeled Navalny's anti-corruption group a "foreign agent", meaning it can be subjected to spot checks and face bureaucratic scrutiny. The term has a negative Cold War-era connotation.

Navalny has called that move and others, including the jailing of several protesters, part of a coordinated and trumped up campaign to stifle the anti-Kremlin opposition's activities.

On Tuesday, investigators said they had seized documents and other items during their searches. Several of Navalny's supporters had been taken in for questioning, they said.