Germany said on Monday it had placed Alexei Navalny under guard in hospital after determining that the long-time critic of Russian President Vladimir Putin had most likely been poisoned while campaigning in Siberia.
Navalny collapsed on a plane on Thursday last week after drinking tea that his allies said they believe was laced with poison. He was flown to Germany for treatment on Saturday.
"The suspicion is that Mr. Navalny was poisoned given that unfortunately recent Russian history has had several such suspected cases," German Chancellor Angela Merkel's chief spokesman, Steffen Seibert, told journalists.
"Because one can say with near certainty that it was a poisoning attack, protection is necessary," Seibert added.
Russia's government made no immediate comment on the German statement. The Kremlin said on Friday it was still unclear what caused Navalny to fall ill and that initial tests did not show he was poisoned.
The incident could further strain Russia's fraught relations with its European and NATO neighbours, who have accused it of mounting attacks on dissidents in Europe in the past - accusations that Russia has dismissed.
Doctors at the Siberian hospital that first treated Navalny said earlier on Monday they had saved his life but that they had not found traces of poison in his system.
"If we had found some kind of poison that was somehow confirmed then it would have been a lot easier for us. It would have been a clear diagnosis, a clear condition and a well-known course of treatment," senior doctor Anatoly Kalinichenko told reporters in the Siberian city of Omsk.
The Russian doctors did not say what they had treated him for. Last week they said they had diagnosed him with metabolic disease possibly brought on by low blood sugar.
The doctors said they had not come under pressure from authorities while treating Navalny.
Navalny's spokeswoman, Kira Yarmysh, said on Monday supporters had reported what they described as a suspected poisoning to the Russian police and Investigative Committee as soon as Navalny fell ill.
The police and Investigative Committee were not immediately available for comment.
Navalny has been a thorn in the Kremlin's side for more than a decade, exposing what he says is high-level graft and mobilising crowds of young protesters.
He has been repeatedly detained for organising public meetings and rallies and sued over his investigations into corruption. He was barred from running in a presidential election in 2018.