Jailed Kremlin critic Alexei Navalny has said he has a persistent cough and temperature, and that there have been cases of tuberculosis in his prison.
Meanwhile, the pro-Kremlin newspaper Izvestia said Navalny, 44, had been transferred to a medical unit with symptoms of a respiratory illness.
Navalny is being held at a penal colony to serve a sentence for embezzlement.
Several people were held outside the jail during a protest against his treatment. He is on hunger strike.
A group of his supporters known as the Alliance of Doctors had gathered at the entrance.
The head of the organisation and Navalny's personal doctor, Anastasia Vasilyeva, tried to gain access to her patient but was rebuffed and then detained by police.
CNN Moscow correspondent Matthew Chance was also among those held.
Just been detained by Russian police outside navalny penal colony pic.twitter.com/aP2N2t80FP— Matthew Chance (@mchancecnn) April 6, 2021
Navalny was given a suspended sentence for embezzlement in 2014 - a conviction widely seen as politically motivated.
The suspended sentence became an actual prison term in February, following his return from Germany, where he had been receiving treatment for poisoning with the nerve agent Novichok.
'Very serious deteriorations'
In an Instagram post (in Russian), the opposition leader said that as many as three people from his unit had been taken to hospital with tuberculosis recently. He said he had a persistent cough and a temperature of 38.1 degrees Celsius (100.6 degrees Fahrenheit).
"If I have tuberculosis, then maybe it'll chase out the pain in my back and numbness in my legs. That'd be nice," he quipped.
But he added that he would be continuing his hunger strike, started last week to demand proper treatment for acute back and leg pain.
The report by Izvestia cited a statement by Russia's prison service, saying Navalny had received various tests, including for coronavirus.
It said his symptoms included a high temperature and that he was being supervised by doctors. However, the statement denied that there was tuberculosis at the colony.
Navalny's lawyers have been visiting him in custody and helping him to post messages on social media. They say there are no doctors at the colony and the medical unit on site is run by a single paramedic.
One of them, Olga Mikhailova, told the independent TV channel Dozhd that Navalny had lost 13kg (28lbs) since he arrived at the prison, including 8kg lost before he started his hunger strike.
She said the authorities were not helping the situation.
"He is completely in the power of the prison service…," she said. "Every day there are very serious deteriorations.
"It's very difficult to understand why this is happening. The prison service is not reacting to a single complaint of ours… There's been complete silence from them in the month since his health got worse."
Russia's prison service last week denied Navalny's allegations that he was not receiving proper treatment, and that he was being deprived of sleep.
It said he had "all the necessary medical assistance in accordance with his medical indications".
The secretary general of leading rights group Amnesty International said she had appealed to Russian President Vladimir Putin over Navalny's "arbitrary arrest and deteriorating health condition".
"There is a real prospect that #Russia is subjecting him to a slow death. He must be granted immediate access to a medical doctor he trusts and he must be freed," Agnès Callamard wrote on Twitter.
The European Court of Human Rights also ruled in February that Navalny should be released because of a risk to his life.
Last week, however, Navalny was visited by Maria Butina, a woman who spent 18 months in custody in the US for working as an unregistered Russian agent there.
In video footage released by Russian state media, she is seen haranguing the politician and describing the prison as better than a hotel.
The BBC's Sarah Rainsford in Moscow says the footage is a clear and co-ordinated attempt by the authorities to discredit Navalny, suggesting that he is lying about the state of his health and is far from a victim of political persecution.