MOSCOW — Russia’s most prominent Kremlin critic, Alexei Navalny, was in critical condition in a German hospital Saturday, suffering from suspected poisoning, as doctors conducted tests after an emergency flight from Russia.

A statement from the Charité hospital in Berlin said a “comprehensive medical exam” was underway on Navalny, 44, who has been in a coma since collapsing early Thursday aboard a plane from the Siberian city Tomsk to Moscow.

His spokeswoman and others suspect he was the target of a deliberate poisoning — a method used before by Russian agents — possibly in tea he drank at an airport cafe. Russian doctors said they did not find toxins in Navalny’s system.

“Once that is complete, and after speaking with the family, the medical staff will provide further details about the sickness and the next steps,” the Berlin hospital statement continued, noting that “the exam will take some time.”

Jaka Bizilj of Cinema for Peace Foundation, the German group that organized Navalny’s flight to Berlin on Saturday, told reporters that Navalny’s condition was “very critical” and “worrying.” Earlier, he told Germany’s Bild newspaper that Navalny was in a stable condition during the flight, which was allowed by Russian officials after previously insisting Navalny was too ill to travel.

Bizilj called Navalny’s move to care in Berlin as “only a small interim success.”

“The decisive question [is] if he survives and survives without lasting damage,” he said.

According to Russian opposition politician and Navalny ally Leonid Volkov, the flight was paid for by the Zimin Foundation of Russian philanthropist and former telecommunications mogul Dmitry Zimin and his son Boris. Dmitry Zimin transferred nearly 90 percent of his fortune to a philanthropic trust, Zimin Trust, in 2007. Both Zimins are involved in the trust.

Volkov also used a Facebook post to thank the German government and Chancellor Angela Merkel for helping “with a huge number of bureaucratic and security issues.”

Volkov wrote that Charité hospital doctors had indicated that it could take days before test results were available.

The Thursday morning flight on which Navalny fell ill was diverted to Omsk, where doctors refused to allow him to be evacuated before finally buckling amid intense international scrutiny.

Navalny’s wife Yulia appealed directly to Russian President Vladimir Putin on Friday in a letter requesting he be allowed to leave.

Navalny is the latest in a succession of Kremlin critics who have fallen mysteriously ill or died with poisoning or suspected poisoning.

On Friday, Alexander Murakhovsky, chief physician at Omsk Emergency Hospital No. 1, said Navalny had not been poisoned but had been diagnosed with a metabolic disease caused by low blood sugar.

Murakhovsky also said traces of industrial chemicals had been found on Navalny’s clothes and hands without offering further explanation.

The Omsk region Ministry of Health announced Saturday that no signs of known toxins were found in Navalny’s system, including barbiturates, strychnine, synthetic poisons and oxybutyrates, used to treat conditions such as narcolepsy.

The ministry said traces of alcohol and caffeine were found in Navalny’s bloodstream. Anastasia Vasilyeva, head of the Alliance of Doctors, an independent group of doctors aligned with Navalny, responded that Navalny never drinks alcohol.

Navalny’s spokeswoman Kira Yarmysh called the refusal to allow Navalny to depart an attempt to take his life, suggesting on Twitter that authorities wanted to thwart an investigation by stalling “until the poison in his body can no longer be traced.”

She said on Saturday that the confirmation that Navalny was in a stable condition throughout the flight only confirmed that nothing had ever prevented Navalny from being evacuated when he urgently needed it.

Volkov said many Navalny supporters had worked for 48 hours in the effort to enable him to leave the country. “What seemed almost impossible even [Friday] morning, when in Omsk they categorically refused to give permission for transport, is now behind us.”

Merkel, French President Emmanuel Macron and British Foreign Secretary Dominic Raab expressed concern Friday about what had happened to Navalny.

Among those who tweeted support for Navalny were actor John Cusack, Democratic presidential nominee Joe Biden, Hillary Clinton and co-host of “The View” Meghan McCain.

President Trump said, “We haven’t seen it yet, we’re looking at it, and Mike [Pompeo] is going to be reporting to me soon.” Former U.S. ambassador to Russia Michael McFaul criticized Trump’s failure to express concern over the case, comparing Navalny to American civil rights leader Martin Luther King Jr., South African freedom fighter and president Nelson Mandela and the leader of the Indian nonviolent struggle against British colonialism Mohandas Gandhi.

“While Navalny has not succeeded yet, there should be no doubt that his cause is good and just,” McFaul wrote, adding, “Vladimir Putin is evil. Over the past 20 years, Russia’s current leader has constructed a ruthless dictatorship.”

He said Trump had shown “not a word of concern, let alone outrage.”

“In the clear divide between good and evil in Russia,” McFaul continued, “Trump is on the wrong side.”

Navalny is Russia’s most prominent opposition figure, known for his scathing YouTube exposés of corruption and graft by Russian politicians, bureaucrats and oligarchs.

He has been frequently jailed for organizing unsanctioned protests. In July, he was forced to shut down his Anti-Corruption Foundation after it was crippled by fines. He pledged to immediately start up a new organization that would do the same work.

In March, authorities froze his bank account and those of all his family members, including his parents, daughter and even his 11-year-old son Zakhar. He was barred from running in a presidential election in 2018.

Among those believed poisoned were crusading journalist Anna Politkovskaya who exposed Russian atrocities in Chechnya and fell ill on a flight in 2004 after drinking tea. She survived but was shot dead outside her apartment in 2006.

Alexander Litvinenko, a former intelligence agent, was poisoned with Polonium-210 while drinking tea in a London hotel. Litvinenko often criticized corruption in Russia under Putin. A British inquiry later found that Putin probably approved his murder.

In 2018, Sergei Skripal and his daughter Yulia nearly died when Russian agents poisoned them with the deadly Soviet-era nerve agent Novichok, or the Newcomer.

Pussy Riot member Pyotr Verzilov, who ran onto the pitch dressed as police with other members of the group at the World Cup soccer final in front of Putin and world leaders in 2018, fell ill with suspected poisoning just weeks later.

Like Navalny, he was evacuated to Berlin’s Charité hospital for treatment. Doctors there said it was “highly plausible” that he had been poisoned.

Other Kremlin opponents, including prominent opposition leader Boris Nemtsov, have been assassinated. Nemtsov was shot dead on a Moscow bridge near the Kremlin in 2015, walking home after eating out.

William Glucroft in Berlin contributed to this report.