Russia's main opposition leader Alexei Navalny has been transferred to a prison hospital, the prison service and his lawyer say, 20 days into a hunger strike that has brought international warnings of consequences should he die in jail.
Allies of Navalny, who have had no access to him since last week, said they were braced for bad news about his health.
They are planning mass countrywide demonstrations later this week, which the Russian authorities have branded illegal.
Navalny's case has further isolated Russia at a time when US President Joe Biden's administration has announced tougher economic sanctions and the Czech Republic, a member of NATO and the European Union, has expelled Russian diplomats, accusing intelligence authorities of a role in deadly 2014 explosions at an arms storage depot.
Russia's prison service said in a statement that a decision had been taken to transfer Navalny, 44, to a regional prison hospital.
His condition was "satisfactory" and he was being given "vitamin therapy" with his consent, it said.
One of Navalny's lawyers, Alexei Lipster, told Reuters he had arrived at the penal colony where Navalny was being held in hospital but had not yet been able to see him.
"Yes, he's here," Lipster said.
"They are not refusing to let me meet him but I have still not been able to get in. I'm waiting."
Ivan Zhdanov, head of Navalny's Anti-Corruption Foundation, said on Twitter the transfer "can only be understood to mean Navalny's condition has worsened, and worsened in such a way that even the torturer admits it".
Navalny, an anti-corruption campaigner who has catalogued the vast wealth accumulated by senior Russian officials he brands "swindlers and thieves," is serving a two and a half year sentence on old embezzlement charges he calls trumped up.
He was arrested on his return to Russia in January after treatment in Germany for what German authorities say was poisoning in Russia with a banned nerve agent.
He and the EU have called this an attempted assassination.
The Kremlin denies any blame.
Navalny went on hunger strike on March 31 to protest against what he said was the refusal of prison authorities to provide him with treatment for leg and back pain.
Russia says he has been treated well and is exaggerating illness to gain attention.