Cholinesterase inhibitors are a broad range of substances that are found in several drugs, but also pesticides and nerve agents. Charité said the specific substance to which Navalny was exposed is not yet known.
"The patient is being treated in intensive care and remains in medically induced coma. While his condition is serious, it is not currently life-threatening," the hospital said in a statement.
Cholinesterase inhibitors act by blocking the breakdown of a key chemical in the body, acetycholine, that transmits signals between nerve cells.
This results in overstimulation of the junction between nerves and muscles. Each year hundreds of thousands of people suffer from cholinesterase inhibitors poisoning, mostly due to exposure to pesticides.
Navalny is being treated with the antidote atropine, the hospital said.
"Alexei Navalny's prognosis remains unclear; the possibility of long-term effects, particularly those affecting the nervous system, cannot be excluded," it said.
The hospital added that it has been in close contact with Navalny's wife, Yulia Navalnaya, who visited her husband in the Berlin hospital.
Navalny, a politician and corruption investigator who is one of Russian President Vladimir Putin's fiercest critics, fell ill on a flight back to Moscow from Siberia on Friday and was taken to a hospital in the city of Omsk after the plane made an emergency landing.
His supporters believe that tea the 44-year-old drank was laced with poison — and that the Kremlin is behind both his illness and a delay in transferring him to Germany.
German authorities posted a special detail of federal agents and city police at the hospital once Navalny arrived out of suspicion he had been the victim of an attack.
"It was obvious that after his arrival, protective precautions had to be taken," Merkel's spokesman, Steffen Seibert, told reporters.
Navalny's team last week submitted a request in Russia to launch a criminal probe, but as of today, Russia's Investigative Committee still has not opened a case, Navalny's spokeswoman Kira Yarmysh said.
Yarmysh pointed out that Navalny's team insisted the politician had been poisoned "from the very beginning, despite statements of the Omsk doctors and state propagandists."
"Now our words have been confirmed by tests in independent laboratories. Navalny's poisoning is no longer a hypothesis, it's a fact," Yarmysh said in a tweet.
Ilya Yashin, an opposition politician in Moscow and a close ally of Navalny, in a video statement Monday urged Russia's law enforcement to investigate "an attempt at the life of a public figure" and to look into the possible involvement of Russian President Vladimir Putin.
"It is Putin who benefits from these endless assaults," Yashin said.
The Kremlin has not commented on the allegation.
- AP
