"I have the right to call a doctor and get medications. They give me neither one nor the other. The back pain has moved to the leg. Parts of my right leg and now of my left leg have lost sensitivity. Jokes aside, but this is already annoying," Navalny said.
Navalny informed the head of penal colony No.2 in Pokrov, in the Vladimir region, that he was going on a hunger strike in a handwritten letter. Images of the letter were shared by his team on Instagram.
"I announce a hunger strike with a demand for the law to be obeyed and that I'm seen by a doctor from outside. So I'm hungry, but so far I still have two legs," Navalny said in the Instagram post.
One of Navalny's lawyers said last week the Russian opposition figure had been suffering from acute back pain that had affected his ability to walk, and his condition was being exacerbated by alleged "torture by sleep deprivation."
Navalny echoed these sentiments on Wednesday, saying he was being tortured. "Instead of medical assistance, I am tortured with sleep deprivation (they wake me up 8 times a night), and the administration persuading the activist convicts (aka "goats") to intimidate ordinary convicts so that they do not clean around my bed," Navalny said.
A group of Russian doctors started an online petition recently calling for prison authorities to allow Navalny to be treated by a doctor from outside of the prison.
The Russian prison service said in a statement on Wednesday that Navalny was receiving all the medical care that he needs and was being treated just like any other convict.
"The procedure for organizing the supervision of convicts, including that of A. Navalny, is carried out in accordance with the requirements of the current legislation and applies to all convicts without exception," the statement from the Russian Federal Penitentiary Service (FSIN) for the Vladimir region said.
"We also inform that convict A. Navalny is being provided with all the necessary medical assistance in accordance with his current medical recommendations."
The statement also addressed Navalny's allegations concerning sleep deprivation, saying prison staff "strictly observe the right of all convicts to a continuous eight-hour sleep. At night, in accordance with the requirements of the law, employees make a detour of residential premises and conduct a visual check of the presence of convicts in the sleeping places. These measures do not interfere with the rest of the convicts."
An outspoken government critic and anti-corruption crusader, Navalny has long been a thorn in President Vladimir Putin's side, prompting concerns for his safety in the country. The activist nearly died after he was poisoned with the Novichok nerve agent last August.
A joint investigation by CNN and the group Bellingcat implicated the Russian Security Service (FSB) in Navalny's poisoning. Russia denies involvement, but several Western officials and Navalny himself have openly blamed the Kremlin. Navalny returned to Russia in January from a five-month stay in Germany, where he had been recovering.
Navalny was jailed earlier this year for violating the probation terms of a 2014 case in which he received a suspended sentence of three and a half years.
A Moscow court took into account the 11 months Navalny had already spent under house arrest as part of the decision and replaced the remainder of the suspended sentence with a prison term last month.