BERLIN — Russian doctors decided late Friday to allow the evacuation of a prominent opposition figure to a German hospital for treatment of suspected poisoning after a day of delays during which they offered a variety of rationales for blocking the transfer.
But late in the day, German doctors who arrived on an air ambulance were permitted to examine the opposition leader, Aleksei A. Navalny, at a Siberian hospital and stated unequivocally that it was safe for him to travel.
Mr. Navalny fell suddenly and violently ill on Thursday on a flight to Moscow from the Siberian city of Tomsk, where he had met with local opposition candidates. The plane was forced to make an emergency landing in the nearby city of Omsk.
Since Mr. Navalny’s arrival at the hospital in Omsk, his family and associates have been bitterly critical of the authorities, who refused to release detailed information on his condition, denied he was poisoned and contended that he was too unstable medically for travel.
The daylong 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,” Mr. Navalny’s spokeswoman, Kira Yarmysh, said.
After the evacuation, Mr. Navalny was expected to be treated at a leading research hospital in Berlin, Charité.
“We were working like crazy through every possible channel to make this happen,” said Jaka Bizilj, a Berlin-based movie producer whose foundation sent the evacuation plane, a Challenger 604 air ambulance. “But I think the breakthrough was the report from the German medical team.”
The foundation, Cinema for Peace, said in a statement that Russian officials’ contention that it was unsafe for Mr. Navalny to travel was “incorrect.”
Mr. Bizilj stressed that the German doctors were not toxicologists and did not give any assessment of what had caused Mr. Navalny’s illness. The producer had previously arranged the transfer of a member of the Russian protest group Pussy Riot, who was also likely poisoned.
Mr. Navalny’s personal doctor, Anastasia Vasilyeva, said in an interview Friday that she believed her patient was poisoned, and that the Russian authorities were delaying Mr. Navalny’s departure long enough for the poison in his system to diminish and become difficult or impossible to identify.
The standoff dragged through the day Friday as Mr. Navalny remained in a coma while the medical evacuation plane sent from Germany waited at the airport.
Mr. Navalny, who is the most persistent critic of President Vladimir V. Putin of Russia, had collapsed in agonizing pain shortly after takeoff on what was to have been a 2,000-mile flight to Moscow. He drank a cup of tea in an airport cafe before departure.
Mr. Navalny’s wife, Yulia, sent Mr. Putin a letter Friday requesting permission to evacuate her husband to Europe for treatment. The Kremlin had earlier said it was open to allowing Mr. Navalny to be flown abroad. But after the German hospital airplane arrived Friday morning, delays ensued.
The head doctor at Hospital No. 1 in Omsk, where Mr. Navalny was being treated, told journalists that he could not release his patient even if relatives requested he do so, because Mr. Navalny’s medical condition was too unstable.
“We cannot allow relatives to take responsibility,” Dr. Aleksandr Murakhovsky said. “Clinical signs raise concerns for us about his transport.”
Dr. Murakhovsky defended a plan under which Mr. Navalny was to be treated in Siberia by Russian specialists from Moscow, who also flew to Omsk overnight. The Moscow doctors, he said, are no worse than the German ones and should be allowed to do their work.
Dr. Murakhovsky, who has a portrait of Mr. Putin in his office and is reportedly a member of the ruling political party, United Russia, also offered the first diagnosis of what had befallen Mr. Navalny on the flight.
“There are working diagnoses,” Dr. Murakhovsky said. “The main working diagnosis which we are most inclining toward is an imbalance in carbohydrates, that is, metabolic disorder. This can be caused by a sharp drop in blood sugar, in the plane, which resulted in loss of consciousness.” He said doctors had found nothing to support the idea that Mr. Navalny had been poisoned.
His contention that Mr. Navalny, an otherwise healthy 44-year-old, had suffered from low blood sugar was quickly dismissed as ridiculous by the opposition leader’s physician, Dr. Vasilyeva.
“This is not a diagnosis,” she said. “If it were just a metabolic disorder he would not be in a coma or on ventilation.” Low blood sugar could be corrected quickly with an injection, she said.
The Kremlin spokesman, Dmitri S. Peskov, then offered another reason for delaying Mr. Navalny’s departure, noting that he had first become ill while on an ascending airplane. If the plane’s ascent had caused the coma, then another flight so soon might “threaten the life of the patient,” he said.
“That’s why the doctor thinks that for now, while there is no clarity about the cause, the patient’s transport by air is impossible.”
Dr. Vasilyeva rejected Mr. Peskov’s theory as “complete nonsense.”
In the interview, Dr. Vasilyeva described a curious encounter earlier reported by Mr. Navalny’s wife, Yulia, and members of his political movement but denied by the Russian authorities.
Dr. Vasilyeva said that she, Yulia Navalnaya and the chief doctor at the Siberian hospital, Dr. Murakhovsky, were discussing treatment when a policewoman entered the room and said Mr. Navalny was poisoned with a substance so lethal it could endanger “those near him.”
Dr. Vasilyeva said the policewoman, who did not introduce herself, had said the substance was “very dangerous” and showed Dr. Murakhovsky the name of the toxin, written on a phone screen. The policewoman said she could not reveal it to others because it was “an investigative secret.”
It was unclear whether this revelation, raising the prospect of a radioactive or chemical threat, was intended as another argument against transporting Mr. Navalny to Germany for treatment. The Russian security services are suspected of using a range of exotic poisons to eliminate opponents, including radioactive polonium 210 and a military nerve agent.
“They are just artificially delaying so no toxic substance will be found in his blood,” Dr. Vasilyeva said. How long this will take, she said, “only they know.”
The Russian authorities have consistently denied any evidence of poisoning. Dr. Murakhovsky, at a news conference Friday, denied this account of the meeting as conveyed by Mr. Navalny’s wife and personal doctor. He said tests for toxins in Mr. Navalny’s blood were all negative.
Melissa Eddy reported from Berlin and Andrew E. Kramer from Moscow.