Doctors have banned Alexei Navalny, the Putin critic who is in a coma after a suspected poisoning, from being transported abroad.
His spokeswoman said doctors had caved to Kremlin pressure but the head doctor said the decision was made because he was not well enough.
A plane had apparently been due to transport him to Germany for treatment following the alleged poisoning.
Russian opposition politician Alexei Navalny went into a coma in a Siberian hospital on Thursday after drinking a cup of tea that his spokeswoman said she believes was laced with poison.
A fierce critic of President Vladimir Putin, Navalny started feeling ill when returning to Moscow from Tomsk in Siberia by plane on Wednesday morning. He was carried off the plane on a stretcher after it made an emergency landing at Omsk.
He was in intensive care and on an artificial lung ventilator in an Omsk hospital, his spokeswoman Kira Yarmysh said.
“We assume that Alexei was poisoned with something mixed into his tea. It was the only thing that he drank in the morning. Alexi is now unconscious,” Yarmysh said.
Doctors gave contradictory information about his condition, saying it had stabilised but also that there was still a threat to his life and they were working to save him.
There is a long history of Kremlin foes being poisoned or falling ill after suspected poisonings.
They include Alexander Litvinenko, who died in London in 2006 after drinking tea laced with polonium-210, and Sergei Skripal, a former double agent who was poisoned with a nerve agent in 2018 in Salisbury, England.
The Kremlin has repeatedly denied involvement in those and other incidents, calling them anti-Russian provocations.
Yarmysh did not say who she believed may have poisoned Navalny but said police had been called to the hospital.
A regional health ministry spokeswoman, Tatyana Shakirova, confirmed that Navalny has been hospitalised in Omsk and said doctors assessed his condition as serious.