MOSCOW -- Russian opposition leader Alexei Navalny was detained on Sunday upon his arrival in Moscow from Germany, where he received medical treatment for alleged poisoning over the past months.

Russia's Federal Penitentiary Service said its officers detained Navalny at the Sheremetyevo International Airport for his "systematic violations of the conditions of the probationary period."

The opposition figure on probation was put on a wanted list on Dec 29, 2020 and he will remain in detention until a court ruling, the service said.

Navalny, a harsh Kremlin critic, fell into a coma on a flight from the Russian city of Tomsk to Moscow on Aug 20, 2020. He was then transferred to a Berlin hospital with suspected poisoning symptoms.

In early September, German Chancellor Angela Merkel said Navalny was poisoned with the Soviet-style Novichok nerve agent.

Russian authorities have repeatedly denied the accusations and demanded solid evidence from Germany.