A Russian hospital is refusing to allow the prominent Kremlin critic Aleksei A. Navalny to be transferred abroad for treatment after a suspected poisoning, the opposition leader’s spokeswoman said on Friday, setting up a potential standoff as a plane from Germany arrived to fly him out of the country.
Mr. Navalny, President Vladimir V. Putin’s most persistent critic, was rushed to the hospital in the Siberian city of Omsk on Thursday after he appeared to suffer agonizing pain on a domestic flight after drinking a cup of tea at the airport in the Siberian city of Tomsk.
Mr. Navalny lost unconsciousness before he was hospitalized. His spokeswoman, Kira Yarmysh, said in a Twitter post on Friday that the head doctor at the hospital in Siberia was barring any transfer because his condition was not stable enough.
The night before, the regional health authorities had said that a group of Russian doctors from Moscow’s most prestigious medical institutions were traveling to Siberia to help treat Mr. Navalny, but the opposition leader’s family and supporters were insisting that he be sent abroad for further treatment.
Ms. Yarmysh wrote that the refusal to allow Mr. Navalny’s transfer was effectively “an attempt on his life being carried out right now by doctors and the deceitful authorities that have authorized it.”
Anatoly Kalinichenko, the deputy head doctor at the Omsk hospital, told reporters on Friday morning that doctors had not found anything to support the idea that Mr. Navalny was poisoned.
He did not explain, however, why a healthy 44-year-old man had fallen ill so suddenly and violently, and there was speculation among Mr. Navalny’s supporters that Russia did not want Mr. Navalny to leave because German doctors might discover the cause of his illness.
Footage from the Moscow-bound airplane on Thursday shared on social media recorded him groaning before losing consciousness, and the plane made an emergency landing in Omsk. He was transported on a gurney to an ambulance waiting on the tarmac.
Doctors at the No. 1 Clinical Hospital in Omsk initially said that Mr. Navalny was on a ventilator in serious condition but later reported that his condition, though still grave, had stabilized.
At the request of Mr. Navalny’s family, the Berlin-based movie producer Jaka Bizilj said his foundation was flying an air ambulance to Omsk and hoped to bring Mr. Navalny to a Berlin hospital, Charité.
Mr. Bizilj said in an interview on Friday that the plane had touched down that morning and was still waiting for the chance to bring Mr. Navalny onboard and take him to Berlin.
“I told the crew to stay there and not to fly back without Navalny,” he said, adding that talks were continuing with the Russian authorities. “Until now it has been a humanitarian initiative, to help save somebody whose life is under threat, but now it seems to be a political thing.”
In 2018, Mr. Bizilj, whose Cinema for Peace Foundation helped produce a documentary about the Russian musical group Pussy Riot, organized a transfer to Berlin for one of the band members, Pyotr Verzilov, after he said he had been poisoned. Mr. Bizilj said that Mr. Verzilov had asked him on Thursday whether he could help arrange the same for Mr. Navalny. He said on Thursday that the biggest concern was whether Mr. Navalny would be stable enough to travel on the five-hour flight.
Ms. Yarmysh, who was traveling with Mr. Navalny in Siberia when he fell ill, has said that he was poisoned, probably by something put into his tea at the airport in Tomsk, and had lapsed into unconsciousness.
As his supporters said Mr. Navalny was fighting for his life, speculation of foul play escalated, particularly after his personal physician and a fellow opposition activist, Anastasia Vasilyeva, was denied access to his medical records and the intensive care ward where he was being treated.
Ms. Yarmysh said on Thursday that Mr. Navalny’s wife, Yulia, had been allowed into his ward after being delayed but that documents needed to fly him out of Omsk to a hospital elsewhere had not been provided.
Chancellor Angela Merkel of Germany and President Emmanuel Macron of France, meeting in France, offered their assistance, including medical help and possible asylum.
The Kremlin initially said it would facilitate Mr. Navalny’s transfer on request, but seemed to back away from this on Friday as doctors in Omsk, who would jump at an order from Mr. Putin to get Mr. Navalny on the plane to Germany, dug in their heels over not letting their patient fly. The mixed messages from the authorities added to suspicions that they want to delay Mr. Navalny’s departure to prevent treatment by foreign doctors who would be more likely to publicly identify any poison remaining in his system.
The Kremlin, which had wished Mr. Navalny a speedy recovery, insisted on Thursday that it was too early to say what caused his illness.
Ms. Yarmysh tweeted that Ivan Zhdanov, the director of Mr. Navalny’s organization, the Anti-Corruption Foundation, had been told by the police in the Omsk hospital that the poison inside him was dangerous not only to the patient but to those around him.
But the police, according to Ms. Yarmysh’s account of what they told Mr. Zhdanov, would not disclose the name of the poison because of “investigation confidentiality,” a term often used in Russia when the authorities want to hide embarrassing or damaging information.
Tass, the state-owned news agency, quoted an unidentified law enforcement source as saying that Mr. Navalny could have “taken something himself” before boarding the plane. But he joined a long list of opponents of Mr. Putin who have been suddenly afflicted by bizarre and sometimes fatal medical emergencies, often after drinking tea.
Melissa Eddy and Andrew Higgins contributed reporting.