Doctors treating Alexei Navalny for a suspected poisoning have refused to release him for evacuation to a clinic in Germany.
Mr Navalny has been in a coma since Thursday when he fell ill on a flight and his supporters called the doctors' decision "a direct threat to his life".
The hospital’s chief doctor, Alexander Murakhovsky, said they did not believe Navalny had been poisoned but declined to name the cause of his sudden illness on Thursday.
He said: "We have a diagnosis, we have complications … I can’t announce the diagnosis that we have made, but it has been told to his wife and the brother of our patient.”
Mr Navalny is a prominent critic of Russian President Vladimir Putin.
His team suspects a poisonous substance was put in his tea at an airport cafe.
Navalny’s press secretary Kira Yarmysh said the decision to delay his evacuation to the European clinic was “an attempt on his life”.
Ivan Zhdanov, the director of Navalny’s Anti-Corruption Foundation, said a police official told him they had found a “deadly substance” in Navalny’s blood, but refused to give any more information as the investigation continues.
“This substance presents a risk to the life of not just Alexei, but those around him too,” the police official had said, according to Zhdanov.
“Everyone around him should be wearing protective equipment.”
Navalny’s team believed hospital officials were stalling for time, possibly until the poison left his system.
The Berlin-based Cinema for Peace Foundation said it had organised a plane to pick up Mr Navalny and bring him back to Berlin, where the Charite hospital was ready to treat him.
Jaka Bizilj, the founder of the German NGO Cinema for Peace Foundation, said: “The plane is in the air, we have all the necessary paper and hope that Alexei is ready for transport tomorrow morning so that we can fly to Berlin.”
The Kremlin had earlier signalled it would authorise the evacuation, despite bans on direct flights to EU countries due to the coronavirus pandemic.
The suspected poisoning has attracted global attention. The White House national security adviser, Robert O’Brien, said the news was “extraordinarily concerning” and could affect US-Russia relations.
If confirmed as a poison attack, it would be the latest in a series of high-profile assaults, often with poison, against opposition figures and Russian dissidents that includes the 2018 poisoning of Sergei Skripal and the 2015 death of Boris Nemtsov, an opposition politician.