Russian opposition leader Alexei Navalny has announced a hunger strike at the penal colony where he is held, to demand proper medical treatment.
He has complained on Instagram of not receiving medical help for back pain and problems with his right leg, as well as being deprived of sleep.
Navalny, 44, returned home from Germany in January after treatment for a nerve agent attack in Siberia in August.
He was taken into custody immediately and then given a jail sentence.
He is currently being detained at the IK-2 corrective penal colony 100 km (60 miles) east of Moscow and has accused the guards there of denying him proper treatment for his ailments.
Russia's federal penitentiary service has rejected his complaint, insisting he had "all the necessary medical assistance in accordance with his medical indications".
A fierce critic of President Vladimir Putin, Navalny was given a two-year and eight month sentence last month for violating the probation terms of an embezzlement case.
The conviction was widely seen as politically motivated and the European Court of Human Rights said he should be freed because of the risk to his life. Kremlin spokesman Dmitry Peskov said this week that Navalny was "responsible for his deeds under the law" and no different from any other Russian.
What has Navalny said?
Navalny's Instagram post on Wednesday, published by his lawyers, shows a handwritten note in which the Kremlin critic writes: "I have declared a hunger strike demanding that the law be upheld and a doctor of my choice be allowed to visit me."
"I have the right to invite a doctor and receive medication. But they are simply not allowing me to do either. The back pain has spread to my leg. I've lost sensation in parts of my right leg and now the left leg too. Jokes aside, this is getting worrying," he wrote.
He said in an earlier post that he had only been given two tablets of the painkiller ibuprofen and was yet to be diagnosed. The penitentiary service (FSIN) said at the time that his health was stable and satisfactory.
Navalny has also complained of being woken up every hour by a guard during the night which he argues amounts to torture. The FSIN said in its statement on Wednesday that staff carried out "a visual inspection of the convicts' presence on their beds" but that did not disturb their sleep.
As well as serious back pain, he spoke of numbness in his leg which was preventing him from standing on it. He thought he was suffering from a pinched nerve after standing "crookedly" in cages during court appearances and travelling in police wagons.
Ned Price, spokesperson for the US state department on Wednesday described Navalny as a "political prisoner" and said his detention was "politically motivated".
"We will continue to seek to hold accountable those in Russia who may be responsible for the attempt on his life, for the repression against his peaceful supporters who have taken to the streets, and we will continue to find ways to support Mr Navalny and to call for his release," he added.