German doctors have begun treating Russian opposition activist Alexei Navalny, who has been in a coma for two days, after what his supporters say was a deliberate poisoning.
After landing at Berlin's Tegel airport on Saturday, Navalny - a major critic of President Vladimir Putin - was taken in a military intensive care vehicle to Berlin's Charite hospital, which lies in the centre of the German capital.
The hospital said it had begun "extensive" diagnostic tests on Navalny. A hospital spokesperson said there would be no official comment on Navalny's status before Monday.
"After completing the examinations and after consulting the family, the physicians will comment on the disease and further treatment steps. The examinations will take some time," the hospital said.
German President Frank-Walter Steinmeier said that finding out why Navalny had fallen into the coma is "critical."
"I hope that all those who can contribute to answering this question will actually do so," Steinmeier said while on a visit to Austria, expressing support for the Russian opposition activist.
"I wish that Mr Navalny recovers not only quickly, but fully, to regain his health so that he can continue working," he said.
Navalny has been in a coma since Thursday. He arrived in Berlin on a German-operated medevac flight from the Siberian city of Omsk.
His spokesperson, Kira Yarmysh, said Navalny was accompanied on the flight by his wife, Yulia Navalnaya, who had also been with him at the hospital in Omsk.
Navalny was in "stable" condition during the flight, activist and film producer Jaka Bizilj, who helped organize the flight, told the Bild newspaper.
Germany's federal government "hopes that the treatment at Charite leads to an improvement in his condition and enables a full recovery," an official spokesperson said.
The transfer to Germany came after the Omsk hospital said their tests did not reveal any evidence of a malicious poisoning.
Blood sugar disorder
The hospital's main theory for Navalny's condition was that he suffered from a metabolic disorder, causing low blood sugar levels.
Navalny, 44, has arguably been the fiercest domestic opponent of Putin for 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 Navalny 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."