Allies of Kremlin critic Alexei Navalny have unveiled plans for what they hope will be the largest protests in modern Russian history as the US warned Russia it would pay a price if he died in jail from his hunger-strike.

The protest date of Wednesday was brought forward after a medical trade union with ties to Mr Navalny said on Saturday he was in a critical condition.

It cited medical tests it said showed that Mr Navalny’s kidneys could soon fail, which could lead to cardiac arrest.

“Things are developing too quickly and too badly,” his allies wrote in a statement on Mr Navalny’s website, announcing their plans for nationwide street demonstrations that they portrayed as a bid to win him life-saving medical care and as a protest over a crackdown on his supporters.

“An extreme situation demands extreme decisions,” they said.

Russian opposition leader Alexei Navalny  was jailed for more than two years for parole violations he maintains were fabricated.

Mr Navalny, a fierce opponent of Russian President Vladimir Putin, started refusing food on March 31 in protest at what he said was a lack of proper medical care for acute back and leg pain provided by prison authorities.

Prison authorities say they have offered Mr Navalny medical care but the 44-year-old opposition politician has refused it and insisted on being treated by a doctor of his choice from outside the facility, a request they have declined.

On Saturday, Mr Navalny’s daughter called on Russian authorities to allow a doctor to treat her father in prison.

“Allow a doctor to see my dad,” Mr Navalny’s daughter Dasha, a student at Stanford University, wrote on Twitter.

Tweet from @Dasha_Navalnaya

A group of actors, writers, historians, journalists and directors, including Jude Law and J.K. Rowling, wrote an open letter to President Putin on Friday urging him to ensure Mr Navalny got adequate medical attention.

A group of opposition regional politicians also called on Mr Putin on Saturday to make sure Mr Navalny was properly treated.

US President Joe Biden’s national security adviser Jake Sullivan said on Sunday that the US had told Russian officials “there will be consequences” if Mr Navalny dies in prison.

European Union foreign ministers are expected to discuss the Navalny case on Monday and Josep Borrell, the bloc’s top diplomat, has also pledged to hold Russia to account.

European Commission President Ursula von der Leyen and German Foreign Minister Heiko Maas called for Mr Navalny to receive immediate medical care as did the US State Department, while French President Emmanuel Macron said world powers should draw “clear red lines” with Russia and consider possible sanctions when they are crossed.

Britain was also deeply concerned by reports of the unacceptable treatment of Mr Navalny and the continued deterioration of his health, the foreign ministry said.

But in a television interview with the BBC, Russia’s ambassador to Britain accused Mr Navalny of attention-seeking.

“He will not be allowed to die in prison, but I can say that Mr Navalny, he behaves like a hooligan, absolutely,” Ambassador Andrei Kelin said in the interview, which was recorded on Friday and aired on Sunday.

“His purpose for all of that is to attract attention for him.”

Mr Navalny has said prison authorities are threatening to put him in a straitjacket to force-feed him unless he abandons his hunger strike.

Russia jailed Mr Navalny for two-and-a-half years in February for parole violations he insisted were fabricated.

He was arrested in January when he returned to Russia from Germany, where he had been recovering from a nerve agent poisoning attack he blamed on Mr Putin.

The Kremlin has said it has seen no evidence he was poisoned and has cast Mr Navalny as a US-backed subversive on a mission to destabilise Russia.

-with AAP