Six doctors, CNN correspondents detained outside Alexei Navalny's prison

Six doctors from the Navalny-backed Alliance of Doctors have been detained in front of the prison in Pokrov where Russian opposition leader Alexei Navalny is currently imprisoned.

Key points: Navalny's supporters demand he be allowed to see a doctor of his choosing

Navalny's supporters demand he be allowed to see a doctor of his choosing His personal physician was refused permission to enter prison grounds or meet local medical staff

His personal physician was refused permission to enter prison grounds or meet local medical staff Three people on Navalny's ward have tuberculosis

Navalny's personal physician Anastasiya Vasilyeva is among the detained people, as well as two CNN correspondents.

It was not immediately clear why any of the detainees were held.

The prison colony IK-2, in Pokrov in the Vladimir region 85 kilometres east of Moscow, stands out among Russian penitentiary facilities for its particularly strict regime.

Allies of Kremlin critic Mr Navalny, 44, said on Friday they would stage a rolling protest outside his prison this week unless he is examined by a doctor of his choice and given what they regard as proper medicine.

Late on Monday, his allies said the protest would go ahead after Mr Navalny said he was continuing his hunger strike, although he had a high temperature and bad cough and three inmates in his ward had been hospitalised with tuberculosis.

Earlier on Tuesday, Dr Vasilyeva had attempted to visit Mr Navalny but was not allowed to enter the prison grounds.

Prison security guards have prevented any of Alexei Navalny's allies entering the facility. ( Reuters: Maxim Shemetov

She said she had applied for a meeting with the head of the prison, however the security in front of the prison did not allow her to meet Mr Navalny or the local medical staff.

"The policemen said that I have no possibility now to visit the chief of this prison. I don't know why," she said.

"All our peaceful actions don't work. It feels like they want to kill Alexei," Dr Vasilyeva said as she was led away.

Police said nine people had been taken into custody for flouting public order.

Authorities stepped up security at the prison early on Tuesday, closing a parking lot to all but prison staff, setting up a makeshift checkpoint and blocking the road.

"It is now under a special (security) regime," a policewoman told Reuters.

Mr Navalny, one of President Vladimir Putin's most prominent critics, has complained of acute back and leg pain and accused authorities of refusing him access to his chosen doctor and of declining to supply him with the right medicine for a condition he has joked darkly could deprive him of the use of both legs.

He was arrested in January upon his return from Germany, where he spent five months recovering from a nerve-agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusation.

Prison authorities say his condition is satisfactory and he has been provided with all necessary medical care, but no imprisoned person can expect any special treatment.