Navalny, a critic of Russian President Vladimir Putin, was flown to Germany for treatment on Saturday after he collapsed on a flight from the Siberian city of Tomsk to Moscow two days earlier.
Berlin's Charite hospital said on Monday that a team of doctors had examined Navalny in detail after his arrival, the Reuters news agency reported.
"The clinical findings indicate poisoning by a substance from the group of active substances called cholinesterase inhibitors," the hospital said in a statement, as quoted by Reuters.
It added that the specific substance was not yet known, but long-term effects, especially to the nervous system, could not be ruled out.
Police patrol the area in front of the hospital where Russian opposition activist Alexei Navalny is receiving treatment in Berlin, Germany, on Monday, August 24, 2020. Photo: EPA/CLEMENS BILAN
Meanwhile, doctors at the Siberian hospital that first treated Navalny said earlier in the day they had saved his life but they had not found traces of poison in his system, Reuters also reported.
Navalny, a lawyer and anti-corruption activist, has been repeatedly detained in Russia in recent years and served several prison terms for leading anti-Kremlin protests.
(gs/pk)
Source: Reuters, PAP