Doctors close to the imprisoned Russian dissident Alexei Navalny, who is currently on hunger strike, demanded Saturday that they be allowed to see him immediately, saying that he could have a heart attack 'at any moment.'

The main opponent of the Kremlin went on a hunger strike on March 31 to protest the poor conditions of detention, and accused the prison administration of preventing him from accessing a doctor and medicine after suffering a double herniated disc, according to his lawyer.

The opposition's 44-year-old personal doctor, Anastasia Vasilieva, and three other doctors, including a cardiologist, asked to see Navalny immediately, according to a letter to the Russian Prison Administration posted on Vasilieva's Twitter account Saturday.

According to her, the level of potassium concentration in Navalny's blood reached a "critical" level of 7.1 mmol / liter, "which means that impaired kidney function and serious heart problems may occur from one minute to the next."

"A patient who has such a level of potassium should be monitored in intensive care, because he may develop a fatal case of arrhythmia at any time. He will die of cardiac arrest," said cardiologist Yarulav Ashikhemin on Facebook.

"It is a complete indication of the need for hospitalization. If the treatment does not start, he will die in the next few days," opposition spokeswoman Keira Ermic said on Twitter.

Alexei Navalny survived a poisoning last year. He says the Kremlin and Russian security services of being behind it.

His wife Yulia, who visited him earlier this week in prison, confirmed that he now weighs 76 kilograms, nine kilograms less than he was at the start of his hunger strike.

More than 70 personalities, including Jude Law, Vanessa Redgrave and Benedict Cumberbatch, demanded in an article published Friday evening in the French newspaper "Le Monde" that necessary medical care be secured for Navalny.

The Russian Public Prosecution demanded Friday that the network of regional offices of Navalny and his anti-corruption institution be classified as "extremist organizations."