BERLIN — Aleksei A. Navalny, the outspoken Russian dissident who fell into a coma last week, was a victim of poisoning, German doctors said Monday, adding him to the ranks of Russians stricken by mystery illness after drawing the wrath of Moscow.
Mr. Navalny, who became ill on a domestic flight in Russia, was under round-the-clock guard at the Berlin hospital where his family transferred him Saturday after what now appears to be yet another attack signaling Russia’s status as an outlaw nation.
While not able to pinpoint the exact poison, German doctors said tests showed it came from a group of chemicals known as cholinesterase inhibitors, which interfere with the nervous system. While they are used medically to treat Alzheimer’s and other forms of dementia, in some forms they are also found in chemical weapons and pesticides.
A Russian health ministry issued a statement challenging the German findings, saying Mr. Navalny’s symptoms were not consistent with cholinesterase inhibitors. But a doctor at the Siberian hospital where the dissident was initially taken said after the German announcement that the patient had been given an antidote often used against nerve agents.
A state news agency, RIA, carried a statement from a pro-Kremlin group that took the opportunity to invoke World War II. The group, Strong Russia, criticized the German government for providing treatment for Mr. Navalny but not for elderly Russians who suffered as children during the Nazi invasion of the Soviet Union.
“Of course, as with any sick person, we wish Navalny a quick recovery,” the statement said. “Has Berlin ever sent an air ambulance for a former detainee in the concentration camps?”
German doctors said that they expected Mr. Navalny to survive. but that it was too early to gauge the long-term effects of the attack.
This time, he fell ill while returning from a trip to Siberia, where he was organizing opposition candidates and strategy for regional and local elections. His plane made an emergency landing in the Siberian city of Omsk, where he was taken to a local hospital.
Mr. Navalny's family and supporters organized an air ambulance to take him to Germany, but Russian doctors delayed for nearly 48 hours, saying his medical condition was too unstable for him to be moved. That drew bitter criticism from the Navalny camp, which accused the doctors of stalling to give the toxins enough time to drain from his system.
Mr. Navalny was flown to Germany at the invitation of Chancellor Angela Merkel. Though Germany enjoys strong economic and cultural ties to Russia, it has not shied from criticizing Mr. Putin’s policies, and even before Mr. Navalny arrived in Berlin, the German government appeared to be taking extra precautions to ensure his safety.
After the doctors declared the case a poisoning, Ms. Merkel and her foreign minister, Heiko Maas, called on Russia to launch an immediate investigation but did not offer any harsher criticism.
“Given Mr. Navalny’s prominent role in Russia’s political opposition, the authorities there are now urgently called upon to investigate this crime to the last detail — and to do so in full transparency,” they said in a statement. “Those responsible must be identified and held accountable."
Minutes before landing, his plane was rerouted from Schönefeld Airport to Tegel Airport, and the ambulance that brought him from the tarmac to the Charité hospital was escorted by the police. A police van and several officers have been stationed outside the hospital’s main entrance since Saturday.
“It was clear that after he arrived here, security measures had to be put in place,” Ms. Merkel’s spokesman, Steffen Seibert, told reporters on Monday, before the hospital released its statement. “We are dealing with a patient who appears, with a certain level of probability, to have been the target of a poisoning attack.”
“Unfortunately,” Mr. Seibert noted, “there are one or more examples of such poisonings in recent Russian history.”
The Russian security services are suspected of having used a range of poisons in attempts to eliminate opponents, although Russian officials have consistently denied it.
Many of those victims have been stricken after drinking tea.
Anna Politkovskaya, an investigative journalist, fell ill after taking a cup of tea on a domestic flight. She survived, but was shot and killed in her apartment elevator two years later.
A former Russian agent turned Kremlin critic Alexander V. Litvinenko, succumbed after ingesting a radioactive isotope, Polonium 210, while having tea with two Russian agents. British investigators later determined that the killing had most likely been ordered by Mr. Putin.
Mr. Navalny, too, drank a cup of tea before falling ill, at an airport cafe before departure.
Numerous, less prominent figures have been felled under mysterious circumstances. In March 2018, a former Russian spy named Sergei V. Skripal was found poisoned on a park bench in Salisbury, England, alongside his unconscious daughter. Both survived.
Sometimes, though, the weapon is not very mysterious.
In 2015, one of the most high-profile Putin critics, Boris Y. Nemtsov, was gunned down just a stone’s throw from the Kremlin. And last August, an assassin riding a bicycle shot and killed a former Chechen rebel commander in a Berlin park; German officials suspect Russia was behind the killing, but Moscow has denied responsibility.
While miscellaneous hit men have been charged in some of the killings, those giving the orders have never been identified.
For the last decade, Mr. Navalny has been Mr. Putin’s most unflinching critic, leading opposition rallies and publishing reports on high-level corruption among Mr. Putin and his cronies — most memorably a lengthy video showing the multiple mega-properties, yachts and other luxuries amassed up by the former prime minister Dimitri A. Medvedev.
Having persevered despite numerous arrests, he likes to call Mr. Putin’s political party the party of “scoundrels and thieves” and has accused the president of trying to turn Russia into a “feudal state.”
Mr. Navalny’s needling criticism of Mr. Putin has never posed a serious electoral threat to the Russian leader, and Mr. Putin remains popular with many Russians. But Mr. Navalny has dominated Russian opposition politics since he led large antigovernment street protests in 2011.
And Mr. Navalny cannily used social media to build a tenacious movement even after much of the independent news media had been squelched and other critics were driven into exile or killed.
Like the German doctors, the Russian doctors who initially treated Mr. Navalny said they had looked at poisoning as the cause for his sudden collapse. Then they ruled it out, they said.
“Of course, we will sort this out,” Anatoly Kalinichenko, the deputy head doctor of the Siberian hospital where Mr. Navalny was treated, said of the German findings. “Did we make a mistake, or did the laboratories, or is all of this disinformation?” he said.
Another doctor who consulted on Mr. Navalny’s treatment in Siberia, Boris Teplykh, said the medical team had in fact considered a poison in the class of chemicals identified by the Germans, which includes nerve agents.
They ruled it out, he said — but before doing so had injected Mr. Navalny with an antidote for nerve toxins, atropine. German doctors said Monday they had also begun atropine treatment.
“Concerning atropine, which our colleagues prescribed for treatment, well, in the first minutes after arrival injections with this substance were made,” Dr. Teplykh told the news agency Interfax.
While Mr. Navalny was still in Russia, the hospital’s head doctor, Aleksandr Murakhovsky, released a statement offering a different diagnosis. He said Mr. Navalny was most likely suffering from a metabolic disorder brought on by low blood sugar.
Men who appeared to be with the security services but were not in uniform had milled about the hospital hallways, and came and went from Dr. Murakhovsky’s office, videos and pictures showed.
Their presence alarmed Mr. Navalny’s wife, his personal physician and a spokeswoman, who said they worried the security services were dictating his care.
Asked about these plainclothes men in his office, Dr. Murakhovsky said that he did not know who they were, but that they had not influenced his treatment decisions.
“I had a lot of people in my office, but I cannot say what they were doing there,” he said. “They came and asked, ‘Is everything all right?’ And I said, ‘Everything is all right.’ And they left.
“They were just interested.”
Melissa Eddy reported from Berlin and Andrew E. Kramer from Moscow.