MOSCOW (AP) — Police on Thursday took into custody two top associates of jailed Russian opposition leader Alexei Navalny ahead of planned protests against his detention.

Navalny, an anti-corruption campaigner who is the Kremlin’s most prominent foe, was arrested on Sunday when he returned to Russia from Germany, where he had spent five months recovering from nerve-agent poisoning that he blames on the Kremlin.

Authorities say his long stay in Germany violated the terms of a suspended sentence in a 2014 conviction for financial misdeeds.

Navalny’s supporters are calling for demonstrations on Saturday.

His spokeswoman Kira Yarmysh was detained by police at her residence several hours after Lyubov Sobol, a lawyer for Navalny’s Fund for Fighting Corruption, was detained. There were no immediate reports of charges lodged against either.

The Moscow prosecutor’s office meanwhile issued warnings to about 30 people against calls for the protests, according to state news agency Tass.

In addition, state communications watchdog Roskomnadzor said it is warning social media outlets that they could face fines of up to 4 million rubles ($53,000) for allowing minors access to calls for unsanctioned demonstrations.

The decision to arrest Navalny upon his return put the Kremlin in a strategic bind — either risk large demonstrations and international denunciations by keeping him behind bars or back down by letting him go free.