Navalny team says Russia street protests set for April 21

Aides to ailing Kremlin critic Alexei Navalny, who is on hunger strike in prison, called on Russians on Sunday to take to the streets on Wednesday to help save his life.

"It's to time to act. We are talking not just about Navalny's freedom but his life," Leonid Volkov said on Facebook as he urged Russians to turn out Wednesday evening, hours after President Vladimir Putin is to deliver his state of the nation address.

He said Wednesday's rally could become a decisive battle against "absolute evil" or Russia's last opposition rally for years to come.

"Call on all your acquaintances and take to central squares," wrote Volkov, who heads Navalny's regional offices, adding that Wednesday's street protests should become massive.

"Don't think that nothing depends on us," he added.

The announcement was released after Navalny's doctors said on Saturday that the opposition politician's health had rapidly deteriorated and he could go into cardiac arrest "any minute".

"Our patient can die any minute," one of Navalny's doctors, cardiologist Yaroslav Ashikhmin said on Facebook on Saturday, pointing to his high potassium levels and saying Navalny should be moved to intensive care.

On March 31, 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 east of Moscow.

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 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.

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