BERLIN — Prominent Russian opposition figure Alexei Navalny was poisoned, Berlin's Charité hospital said in a statement Monday, citing clinical results that contradicted Russian doctors and corroborated claims that an attempt had been made on the Kremlin critic's life.

Although the exact substance that poisoned Navalny is not yet known, it is believed to be a cholinesterase inhibitor, Charité’s statement said, adding that the effect of the toxin — blocking cholinesterase, an enzyme needed for the proper functioning of the nervous system — was confirmed several times by independent laboratories.

The hospital said that “another broad analysis has been initiated” and that Navalny remains in a medically induced coma but “there is no acute danger to his life.” He is being given atropine, a medication used to treat certain types of nerve agent and pesticide poisonings.

“Longer-term effects, especially in the area of the nervous system, cannot be ruled out,” the statement said.

Prominent Kremlin critic Alexei Navalny arrived in Berlin on Aug. 22. There he was in a critical condition, undergoing tests at a German hospital. (William Glucroft/The Washington Post)

Navalny, 44, was stricken Thursday during a flight to Moscow from Siberia. His spokeswoman and others quickly asserted that he was the latest victim of a state-ordered poisoning, a method used before in attacks linked to Russian agents by Western intelligence officials and others.

One such attack also featured a cholinesterase inhibitor: The deadly Soviet-era nerve agent known as Novichok was used in the poisoning of Sergei Skripal, a former Russian spy, and his adult daughter, Yulia, in Britain two years ago.

Cholinesterase inhibitors are found in drugs used to treat some diseases, including Alzheimer’s, but certain classes can be used in pesticides and as nerve agents for biowarfare, according to the National Institutes of Health. The chemical can be delivered orally; Navalny’s spokeswoman, Kira Yarmysh, said she suspected the tea he purchased at an airport cafe Thursday morning was laced with a toxin because it was the only thing she saw him ingest that day.

Atropine is the most common treatment for poisoning by a cholinesterase inhibitor. Patients who are managed immediately after poisoning tend to respond positively, but delays in treatment can lead to adverse outcomes, according to the National Institutes of Health.

Navalny was loudly moaning in pain and then lost consciousness during Thursday’s flight to Moscow, prompting the pilot to make an emergency landing in Omsk, where Navalny spent two days in a hospital. At the urging of his family, Omsk doctors approved Navalny’s release on Friday so he could be flown to Germany aboard an ambulance aircraft the next morning.

Charité hospital’s conclusion contradicted doctors in Omsk, who repeatedly said there was no evidence that Navalny was poisoned. One diagnosis they suggested was that Navalny’s condition was caused by a steep drop in blood sugar.

“The poisoning diagnosis was one of the first to be suggested, including by paramedics,” Anatoly Kalinichenko, the deputy chief physician of Omsk Emergency Hospital No. 1, told reporters Monday. “This is why the patient was taken to the toxicology department. If we had found any confirmation of poisoning, things would have been much easier for us.”

“We received definitive answers from two laboratories, which said they did not detect any chemical or toxic substances they could describe as poisons or poisoning products,” Kalinichenko added.

Navalny’s associates were critical of his treatment last week by the medical staff in Omsk, accusing physicians of initially blocking his move to Berlin because they faced pressure from authorities seeking to hinder investigation into the incident.

Chief doctor Alexander Murakhovsky said at a news conference Monday that the hospital “made quite an effort to save Alexei Navalny’s life, there can be no doubt about it.”

Asked about a photo posted on Twitter by Yarmysh, Navalny’s spokeswoman, showing three unidentified men in plain clothes sitting in his office — men who Yarmysh claimed were members of Russia’s security services — Murakhovsky said: “I can’t say who that was.”

Since arriving at Charité, Navalny has been under the protection of Germany’s Federal Criminal Police Office, which also provides security for German Chancellor Angela Merkel and other government officials. That protection doesn’t usually extend to private citizens, but German government spokesman Steffen Seibert said Monday that an offer to help had been extended on “humanitarian grounds” and at the request of Navalny’s family.

“It was clear after his arrival that security measures had to be put in place because we are dealing with a patient who was likely the target of a poison attack,” Seibert said, adding that other Kremlin critics have been poisoned in recent years.

Pyotr Verzilov, a member of the Pussy Riot protest group, accused Russian military intelligence of responsibility for his suspected poisoning in 2018. After being initially hospitalized in Moscow, he was also transferred to Charité for treatment. No traces of poison were found in his system, but hospital staff said there was no other explanation for his condition.

Navalny, who was barred from running for president in 2018, has no shortage of ill-wishers in Russia. For nearly a decade, his Anti-Corruption Foundation produced written and online video reports exposing graft and other wrongdoing by President Vladimir Putin’s allies. Last year, Navalny and his team launched its “Smart Vote” initiative, which helps voters identify which candidates have the best chance of beating out those aligned with Putin’s United Russia party.

“Given Mr. Navalny’s prominent role in Russia’s political opposition, the authorities there are now urgently called upon to clarify this act to its fullest, and with complete transparency,” Merkel and German Foreign Minister Heiko Maas said in a joint statement. “Those responsible must be investigated and held to account.”

Khurshudyan reported from Moscow.