Russian police detained opposition leader Alexei Navalny, an outspoken critic of President Vladimir Putin, as he arrived in Moscow after being treated in Germany for poisoning, an arrest that is set to pose an early test for U.S. President-elect Joe Biden’s incoming administration.

Navalny, 44, was met by officers at passport control as he landed in Moscow on a plane from Berlin Sunday, according to a live video feed on his YouTube channel. The Federal Penitentiary Service said he had been detained for violating the terms of a suspended sentence, state-run Tass reported.

Navalny, whose anti-corruption exposes and success in galvanizing anti-government votes have increasingly needled the authorities, had been recovering in Germany from a nerve-agent attack in August that he and Western governments blamed on Putin.