Russian opposition leader and anti-corruption activist Alexei Navalny has been moved from prison in Moscow to an unknown destination, aides say.
He was taken from the remand prison without any notification being given to his supporters and may have been sent to a prison camp, they added.
Earlier this month a Moscow court converted a 2014 suspended sentence for embezzlement into a prison term.
The original conviction was widely seen as politically motivated.
Navalny is the most ferocious critic of the Russian authorities under President Vladimir Putin.
Last summer he was poisoned in Siberia with a chemical nerve agent. An independent investigation has alleged that a hit squad of Russian security agents tried to kill him.
In a coma he was airlifted to Germany, where he recovered. He returned to Russia in January and was arrested on arrival.
The court this month found that he had violated the terms of his probation over the embezzlement case, and imposed the sentence of nearly three years.
In another development, EU leaders condemned Navalny's treatment on Thursday and demanded his immediate release.
Where is he now?
Navalny lawyer Vadim Kobzev said he had arrived at the remand prison to meet his client only to be told that he was no longer there.
According to Mr Kobzev, at the prison they would not tell him to which prison camp the opposition activist had been taken.
Eva Merkacheva, a member of Moscow's public commission that monitors detainees' human rights, told AFP news agency she was confident Navalny had been sent to a penal colony.
"There are just no other options," she said, adding that by law Navalny should serve his sentence in a prison not far from the capital.
Meanwhile, a row over Amnesty International's decision to remove "prisoner of conscience" status from Navalny, on the grounds that he made xenophobic comments in the past, took another turn.
Russian pranksters announced they had tricked Amnesty officials into admitting that the move had "done a lot of damage."
Vovan and Lexus, who for years have been fooling Western politicians, released a recording of a 14-minute video call with Julie Verhaar, Amnesty's acting secretary general, and two other directors.
One of the pranksters posed as Leonid Volkov, Navalny's right-hand man.
The real Leonid Volkov responded by tweeting, "Frankly - and I hate to say that - this Zoom call alone is, in my opinion, enough to qualify the @amnesty leadership as unfit."
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