Tests indicate that leading Russian opposition figure Alexei Navalny was poisoned, the Berlin hospital where he is being treated in serious condition said on Monday.
Charite hospital said the precise toxin remains unknown, but they assume the substance came from a group of cholinesterase inhibitors, a chemical that prevents the breakdown of certain neurotransmitters.
While 44-year-old Navalny's state of health was serious, there was no acute danger to his life, a spokesperson said.
She said the prognosis remained uncertain, and long-term consequences, in particular for the nervous system, could not be ruled out at this stage.
Navalny was taken ill on a flight from the Siberian city of Tomsk to Moscow on Thursday. He was flown to Germany on Saturday for emergency treatment at Charite, the capital's leading hospital.
He has been in a coma and was initially treated in a hospital in Omsk, Siberia before doctors there agreed, following a lengthy wrangle, to allow him to travel to Germany.
The hospital's findings support the claims of Navalny's team, which has pointed to poisoning at the likely cause from the outset. They have suggested the tea he drank at an airport cafe in Tomsk may have been spiked.
Navalny's wife, Julia, arrives at the Charite University Hospital. Photo: Kay Nietfeld/dpa.
Russian hospital blamed blood sugar level
The Russian hospital on Omsk had said there appeared to be no evidence of a malicious poisoning and suggested that Navalny could have gone into a coma due to low blood sugar levels.
Navalny has been the face of domestic opposition to Russian President Vladimir Putin, organizing several series of protests against the long-time Russian leader, whom he accuses of perpetuating widespread corruption.
Navalny had been in Siberia to support opposition candidates preparing for local elections.
Earlier, the Omsk hospital said it had not been under any political pressure from state authorities.
"We saved his life. There was no outside influence on the care for the patient and there couldn't have been," the head doctor at the hospital in Omsk, Alexander Murakhovsky, said in comments carried by the Russian news agency Interfax.
Murakhovsky said that any major decision about the Russian hospital's care for Navalny had been decided by a council of up to 10 doctors representing various medical institutions.