Berlin - German doctors said on Monday that medical examinations indicated that Russia opposition figure Alexei Navalny, who is in a Berlin hospital after collapsing on a plane in Russia last week, had been poisoned.
Navalny, a critic of Russian President Vladimir Putin, had been flown to Germany for treatment on Saturday.
The Kremlin has said it was unclear what caused Navalny to fall ill and that initial tests did not show he was poisoned, as his aides charged.
Berlin's Charite hospital said a team of doctors there had examined him in detail after his arrival.
"The clinical findings indicate poisoning by a substance from the group of active substances called cholinesterase inhibitors," the hospital said in a statement.
The specific substance was not yet known, they said. The outcome remains uncertain but long-term effects, especially to the nervous system, could not be ruled out, it said.
Cholinesterase inhibitors are drugs that can increase communication between nerve cells in the brain. They are sometimes used to temporarily improve or stabilise the symptoms of people with dementia.
Common side effects of cholinesterase inhibitors include vomiting, muscle cramps, headache and hallucinations.
Certain chemical classes of pesticides work against bugs by interfering with, or 'inhibiting' cholinesterase but they can also be poisonous, or toxic, to humans in some situations.
Navalny collapsed on a plane on Thursday last week after drinking tea while on his way to campaign in Siberia.
Doctors at the Siberian hospital that first treated Navalny said earlier on Monday they had saved his life but they had not found traces of poison in his system. They had not come under pressure from authorities while treating Navalny, they said.
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.