Leading Kremlin critic Alexei Navalny, who has suffered a suspected poisoning, was in a stable condition in hospital on Saturday after being flown to Berlin following a standoff over his medical evacuation from Russia.

A convoy including two yellow ambulances brought Navalny from Berlin's Tegel airport to the renowned Charite hospital just after 10:20 am local time (0820 GMT).

Ambulance transports Navalny from Omsk hospital to airport
Ambulance transports Navalny from Omsk hospital to airport AFPTV / Andrey BORODULIN

"Navalny's condition is stable," Jaka Bizilj, the head of the Cinema for Peace foundation that brought Navalny to Germany in a chartered medical plane, told AFP.

Berlin's Charite hospital confirmed in a statement that it had admitted Navalny and was carrying out an "extensive medical diagnosis".

The plane took off on Saturday morning to fly Navalny to Berlin's Charite hospital for treatment
A representational image of a plane. AFP / Dimitar DILKOFF

The 44-year-old lawyer and anti-corruption campaigner, one of President Vladimir Putin's fiercest critics, went into a coma after falling suddenly ill Thursday on a plane to Moscow that had to make an emergency landing in Omsk.

Aides say they believe Navalny was poisoned, apparently via a cup of tea at the airport, and blamed Putin, though Russian doctors said tests showed no trace of any poison.

Doctors treating him in Omsk initially refused to let Navalny leave but reversed course after his family and staff demanded he be allowed to travel to Germany
Doctors treating him in Omsk initially refused to let Navalny leave but reversed course after his family and staff demanded he be allowed to travel to Germany AFP / Dimitar DILKOFF

Doctors treating him in Omsk had refused to let Navalny leave but reversed course after his family and staff demanded he be allowed to travel to Germany.

As the plane left Omsk, Navalny's wife Yulia thanked supporters via Instagram for their "persistence".

"Without your support, we wouldn't have been able to take him!" she wrote.

Navalny is the latest in a long line of Kremlin critics who have fallen seriously ill or died in apparent poisonings
Navalny is the latest in a long line of Kremlin critics who have fallen seriously ill or died in apparent poisonings AFP / Dimitar DILKOFF

The Omsk regional health ministry said Saturday that both caffeine and alcohol were found in Navalny's urine, but "no convulsive or synthetic poisons were detected".

Navalny's spokesman Kira Yarmysh said he had neither drunk alcohol nor taken any medication.

Navalny has made many enemies with his anti-corruption investigations
Navalny has made many enemies with his anti-corruption investigations AFP / Kirill KUDRYAVTSEV

The air ambulance arrived in Omsk on Friday morning but Russian doctors initially said Navalny was too "unstable" to be moved, before relenting later that day.

High-profile poisonings
High-profile poisonings AFP / Alain BOMMENEL

Navalny's wife had appealed directly to Putin to let him leave, while his aides asked the European Court of Human Rights to intervene.

Human rights group Cinema for Peace said it had financed the medical transport with private money, although its chief Bizilj would not reveal the list of donors it cobbled together at the last minute.

Russian opposition figure Alexei Navalny was expected to  to be treated at Berlin's Charite hospital
Russian opposition figure Alexei Navalny was expected to to be treated at Berlin's Charite hospital AFP / Odd ANDERSEN

Navalny is the latest in a long line of Kremlin critics who have fallen seriously ill or died in apparent poisonings.

His wife told journalists that she wanted Navalny to be "in an independent hospital, whose doctors we trust".

Yarmysh tweeted that "the battle for Alexei's life and health is just beginning... but at least now we've taken the first step."

The air ambulance was dispatched to take Navalny to Berlin after Chancellor Angela Merkel extended an offer of treatment.

European Union leaders including Merkel and French President Emmanuel Macron have voiced concern for Navalny, who has faced repeated physical attacks and prosecutions in more than a decade of opposition to Russian authorities.

Navalny lost consciousness shortly after his plane took off on Thursday from Tomsk in Siberia, where he was working to support opposition candidates ahead of regional elections next month.

Yarmysh said he had seemed "absolutely fine" before boarding the flight and had only consumed a cup of tea at the airport.

She said she was sure he had suffered from an "intentional poisoning" and blamed Putin.

She also claimed Russia's refusal to evacuate Navalny was a ploy to "play for time" and make it impossible to trace poison, posing a "critical threat to his life".

Navalny has made many enemies with his anti-corruption investigations, which often reveal the lavish lifestyles of Russia's elite and attract millions of views online.

The director of the Anti-Corruption Foundation that Navalny founded, Ivan Zhdanov, confirmed on social media that the organisation was "continuing its work".

Many supporters expressed relief he was going for treatment outside Russia.

"I feel as relieved now as if terrorists had freed a hostage after long negotiations," fellow opposition politician Ilya Yashin tweeted, criticising the delay in Navalny's departure.