Berlin (dpa) - Russian opposition activist Alexei Navalny, who has been in a coma since earlier this week, arrived in Germany on Saturday for treatment for what his supporters say was a deliberate poisoning.

After landing at Berlin's Tegel airport, he was taken in a military intensive care vehicle to the Charite hospital in the centre of the German capital.

His spokesperson Kira Yarmysh said Navalny was accompanied on the German-operated flight by his wife, Yulia Navalnaya, who had also been with him at the hospital in the Siberian city of Omsk.

He had been in "stable" condition during the flight, activist and film producer Jaka Bizilj, who helped organize the flight, told the Bild newspaper.

The transfer to Germany came after the Omsk hospital said their tests did not reveal any evidence of a malicious poisoning.

The hospital's main theory for Navalny's condition was that he suffered from a metabolic disorder, causing low blood sugar levels, head doctor Alexander Murakhovsky told state media.

Navalny, 44, has arguably been the fiercest domestic opponent of President Vladimir Putin over the past decade, having organized several series of protests against the long-time Russian leader, whom he accuses of perpetuating widespread corruption.

Navalny had spent nearly two days in the Omsk hospital before Russian authorities granted permission for him to be transported out of the country, saying his condition had stabilized. They had earlier refused Germany's offer, saying it would have been too risky to move him.

Yarmysh claimed that Russian authorities had delayed Navalny's departure as part of a cover up to hide the real reason for his illness.

Yarmysh has said that a toxic substance must have been added to the tea that he drank at an airport cafe before boarding a plane in the Siberian city of Tomsk.

Navalny had been in Siberia to support opposition candidates for local elections. He fell ill while on the flight from Tomsk back home to Moscow, and the plane made an emergency landing in Omsk.

German Chancellor Angela Merkel has demanded rapid clarification from Russia as to the circumstances of Navalny's illness.

The European Union has called for an "independent and transparent investigation."