Jailed Kremlin critic Alexei Navalny risks cardiac arrest at "any minute" as his health has rapidly deteriorated, doctors warned Saturday, urging immediate access to Russia's most famous prisoner.

On March 31, President Vladimir Putin's most prominent opponent went on hunger strike to demand proper medical treatment for back pain and numbness in his legs and hands.

Navalny, 44, was imprisoned in February and is serving two-and-a-half years on old embezzlement charges in a penal colony in the town of Pokrov around 100 kilometres (60 miles) east of Moscow.

Navalny's personal doctor Anastasia Vasilyeva and three more doctors including cardiologist Yaroslav Ashikhmin have asked prison officials to grant them immediate access.

"Our patient can die any minute," Ashikhmin said on Facebook on Saturday, pointing to the opposition politician's high potassium levels and saying Navalny should be moved to intensive care.

"Fatal arrhythmia can develop any minute."

Navalny barely survived a poisoning with the Novichok nerve agent in August which he has blamed on the Kremlin. His doctors say his hunger strike might have exacerbated his condition.

Having blood potassium levels higher than 6.0 mmol (millimole) per litre usually requires immediate treatment. Navalny's were at 7.1, the doctors said.

"This means both impaired renal function and that serious heart rhythm problems can happen any minute," said a statement on Vasilyeva's Twitter account.

The doctors said he had to be examined immediately "taking into account the blood tests and his recent poisoning".

"Alexei is dying"

Navalny's spokeswoman Kira Yarmysh, who accompanied him when he collapsed on a plane after the poisoning in August, said the situation was critical again.

"Alexei is dying," she said on Facebook. "With his condition it's a matter of days."

She said she felt like she was "on that plane again, only this time it's landing in slow motion", pointing out that access to Navalny was restricted and few Russians were aware of what was actually going on with him in prison.

More than 70 prominent international writers, artists and academics, including Jude Law, Vanessa Redgrave and Benedict Cumberbatch, have called on Putin to ensure that Navalny receives proper treatment immediately.

Their appeal was published late Friday by France's Le Monde newspaper.

Navalny's team has earlier announced plans to stage what they said would be "modern Russia's biggest protest".

Navalny's allies said they would set a date for the protest once 500,000 supporters had registered with a website. As of Saturday, more than 440,000 people had signed up.

Yarmysh on Saturday urged more Russians to sign up, saying that a big rally could help save Navalny's life.

"Putin only reacts to mass street protests," she added.

Earlier this week, Navalny's wife Yulia, who visited him in the penal colony, said her husband now weighed 76 kilograms (168 pounds) -- down nine kilograms since starting his hunger strike.

On Friday, Russian prosecutors asked a court to label Navalny's Anti-Corruption Foundation and the network of his regional offices "extremist" organisations in a move that would outlaw them in Russia and could result in jail time for their members.

"The darkest times are beginning for free-thinking people, for civil society in Russia," said Leonid Volkov, the head of Navalny's regional offices.