Russian police arrested supporters of Alexei Navalny on Tuesday, including doctors, in front of the high-security prison where the hunger-striking Kremlin critic is being held.
The delegation had made the trip to demand that Navalny gets qualified medical help from independent doctors.
The 44-year-old has complained about pain in his leg and back and on Monday it was suggested that he had a fever. Navalny had previously said that a number of patients in the prison have tuberculosis.
Anastasia Vasilyeva, the leader of the Alliance of Doctors, arrived at prison colony IK-2 on Tuesday.
According to AFP, she is among the people who were arrested in front of the prison, which is located 100 km east of Moscow.
In an Instagram post, Navalny said that three people in his section had been hospitalised with TB and complained about the conditions in the prison camp.
"Inside there are unsanitary conditions, tuberculosis, lack of drugs," he wrote, stating he was "genuinely surprised there is no Ebola".
He criticised Russian state media, which has aired claims in recent days that the prison in which he is being held was "ideal" and even had its own restaurant.
"I invite the correspondents of Putin's channels to spend the night in our 'ideal prison' on the next bed with a coughing convict."
Navalny said he had a severe cough and a temperature of 38.1, which is considered a fever.
Life in danger
On April 1, an ally of Navalny, Vladimir Ashurkov, executive director of the Anti-Corruption Foundation, said that the opposition leader's life was in danger.
Navalny was arrested in January when he returned from Germany, after receiving treatment after being poisoned with a nerve agent, an attack he blamed on the Kremlin. Moscow has denied the claim.
Ashurkov speculated that the pain Navalny was feeling in his back and leg was linked to the attack.