BERLIN (NYTIMES) - Russia's most prominent opposition figure Alexei Navalny arrived in Berlin for treatment on Saturday (Aug 22) after falling into a coma in Siberia in what his family and supporters suspect was a deliberate poisoning weeks before nationwide local elections.

Mr Navalny was admitted to Charite, one of Germany's leading medical research facilities, where he was undergoing extensive diagnostic tests, the hospital said after a plane transporting him from Russia touched down. He arrived more than 48 hours after he first lost consciousness, a delay his supporters bitterly criticised Russian officials for having caused.

"Patient stable, mission accomplished," said Mr Jaka Bizilj, who runs the Cinema for Peace, the foundation that organised the air transport at the urging of Mr Navalny's friends and family.

Mr Navalny became violently ill last Thursday shortly after a Moscow-bound flight he had boarded took off, forcing an emergency landing in the Siberian city of Omsk.

The sudden illness struck Mr Navalny, who is the most persistent critic of President Vladimir Putin of Russia, just as popular uprisings have sprung up in Russia's Far East and in neighbouring Belarus - and only weeks before Russians vote in municipal elections on Sept 13.

How it will affect the political scene, analysts in Russia said, will hinge in part on Mr Navalny's longer-term condition and whether he will be able to return home.

"Sometimes, instances that are publicly perceived as political terror do demoralise the opposition," Ms Ekaterina Schulmann, a Moscow-based political analyst, said in an interview. "And at other times, they motivate people to protest or, at the least, to vote in protest."

If Mr Navalny remains in Germany for a lengthy recovery, or indefinitely as a political exile, the Kremlin stands to benefit politically, she said.

"It is very useful to have an opposition figure in exile," Ms Schulmann said. "He can be cast in the state media as a person who fled Russia. They can present it as unpatriotic behaviour."

In the days before he fell ill, Mr Navalny had been meeting with opposition candidates in Novosibirsk, Siberia's largest city, promoting a strategy he called "smart voting" that encourages multiple, small opposition movements to back a single candidate on a local ballot.

The strategy seeks to chip away at the dominance of the pro-government party, United Russia, in city councils and regional parliaments.

President Putin's popularity has been in decline for the past two years as nationalist fervour over the annexation of Crimea from Ukraine has faded and as the economy slumped under sanctions and then coronavirus lockdowns.

Upon landing in Berlin, Mr Navalny's plane was met by an ambulance that took him, under police escort, to the hospital where doctors began extensive testing to determine what may have caused the illness and how to treat it. Several police officers were stationed outside the hospital throughout the day.

The examinations, said Ms Manuela Zingl, a spokesman for Charite, "will take some time".

Mr Navalny collapsed in agonising pain last Thursday shortly after take-off on what was to have been a 3,220km flight to Moscow. His family suspects that poison may have been added to a cup of tea he drank in the airport before boarding that flight.

His evacuation came only after long hours of wrangling with Russian doctors and officials, who had insisted that a transfer to Germany would endanger Mr Navalny's health. But German doctors, who had arrived in Omsk on the air ambulance, were granted access to Mr Navalny last Friday afternoon, and they stated unequivocally that it was safe for him to travel.

Mr Navalny's wife, Yulia, who had sent Mr Putin a letter last Friday requesting permission to evacuate her husband, was allowed to accompany him to Germany.

The Russian authorities said tests for toxins in Mr Navalny's blood were all negative, indicating they found no evidence he was poisoned. At a news conference last Friday, Dr Alexander Murakhovsky said Mr Navalny had suffered an "imbalance in carbohydrates, that is, metabolic disorder", possibly caused by low blood sugar.

Mr Navalny's wife and personal doctor quickly dismissed this account, saying the idea that an otherwise healthy 44-year-old would collapse and fall into a days-long coma from low blood sugar was ridiculous.

If Mr Navalny is found to have ingested dangerous toxins, he would become the latest prominent Kremlin critic to have been the victim of a poisoning.

A fatal dose of the radioactive substance polonium 210 was used against Mr Alexander Litvinenko, and a nerve agent called Novichok against Mr Sergei Skripal, both former Russian intelligence officers attacked in England.

Former Ukrainian president Viktor Yushchenko fell ill from a dioxin, and Mr Vladimir Kara-Murza, a Russian journalist who lobbied in the West for sanctions against Kremlin operatives, said unknown toxins were used against him.

The people who ordered the attacks have never been identified, despite efforts to take cases to higher courts, including the European Court of Human Rights.

Officials in Berlin did not immediately comment on Mr Navalny's arrival. But in offering this past week to allow the opposition leader into Germany for medical treatment, Chancellor Angela Merkel called for a thorough investigation.

"What is particularly important is that the circumstances behind this are cleared up very quickly," Dr Merkel said. "We insist on this, because what we have heard so far is very unfavourable."

Mr Navalny has been treated abroad previously, after an attack in 2017 in which a caustic liquid was thrown in his face, partially blinding him in one eye. He resumed his political activities almost without interruption. Dozens of arrests and short spells in jail have also failed to deter him.

Still, violence by pro-Kremlin activists and arrests for administrative offences have kept him out of sight for important events like protests and elections. Mr Navalny will now be absent from campaigning before the September local elections.