Kremlin critic Navalny could 'die any minute': doctors
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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.
On Saturday, US President Joe Biden added his voice to a growing international chorus of protest at the treatment of the activist, describing his situation as "totally unfair".
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.
"Fatal arrhythmia can develop any minute."
"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.
On Saturday, responding to reporters' questions about Navalny's plight, Biden responded: "It's totally, totally unfair, totally inappropriate".
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 had 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 2230 GMT Saturday, more than 450,000 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.