By Andrew Osborn and Gabrielle Tétrault-Farber
MOSCOW (Reuters) -Allies of Kremlin critic Alexei Navalny unveiled plans on Sunday for what they hope will be the largest protests in modern Russian history on Wednesday as Washington warned Russia it would pay a price if he died in jail from his hunger-strike.
The protest date was brought forward after a medical trade union with ties to Navalny said on Saturday he was in a critical condition, citing medical tests which it said showed that 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 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.
The fate of 44-year-old Navalny, one of President Vladimir Putin's most prominent critics, is adding to already acute strains in Russia's ties with the West. President Joe Biden's national security adviser Jake Sullivan said on Sunday that the U.S. government had told Russia "there will be consequences" if Navalny died in prison.
European Union foreign ministers are expected to discuss the Navalny case on Monday, and Josep Borrell, the bloc's top diplomat, pledged to hold Moscow to account over the matter too.
European Commission President Ursula von der Leyen and German Foreign Minister Heiko Maas called for Navalny to receive immediate medical care as did the U.S. 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.
London was also deeply concerned by reports of the unacceptable treatment of Navalny and the continued deterioration of his health, said Britain's foreign ministry.
The authorities have broken up pro-Navalny protests by force in the past, detaining thousands, and Wednesday's planned demonstration falls on the same day as President Vladimir Putin gives a state-of-the-nation speech.
That sets up a showdown that Navalny's allies described as the last chance to stop Russia from sinking into "darkness".
"A really tough final battle between normal people and absolute evil lies ahead," they said.
Russian authorities accuse Navalny of exaggerating his medical condition to grab attention, and of refusing prison medical care. They have pledged to ensure he survives.
"He will not be allowed to die in prison, but I can say that Mr Navalny, he behaves like a hooligan," Andrei Kelin, Russia's ambassador to Britain, said in a BBC interview on Sunday.
Navalny has said prison authorities are threatening to put him in a straitjacket to force-feed him unless he accepts food. Some activists have called for him to be flown out of the country for emergency medical care.
Navalny travelled to Germany last August for treatment following a nerve agent poisoning attack he blamed on Putin. He was arrested in January when he returned to Russia and jailed for 2-1/2 years in February for parole violations he said were fabricated.
The Kremlin has said it has seen no evidence he was poisoned and has denied any Russian role if he was.
The authorities cast Navalny and his supporters as U.S.-backed subversives bent on destabilising Russia and moves are under way to declare them extremists, opening the door to long jail terms.
Navalny began refusing food on March 31 in protest at what he said was the refusal of prison authorities to provide him with adequate medical care for acute back and leg pain.
Prison authorities say Navalny was offered proper medical care but refused, insisting on being treated by a doctor of his choice from outside the facility, a request they have declined.
Navalny's allies say he refused prison medical treatment because it is outdated and, at times, even dangerous.
'LIFE HANGING BY THREAD'
Navalny's allies had declared a moratorium on protests after staging three demonstrations at the height of winter. Some protesters were unhappy that the demonstrations were halted, but organisers said they would hold a big protest once 500,000 people had registered online to take part.
In light of Navalny's poor health, organisers said they were calling the protest on Wednesday anyway despite being around 40,000 people short of their target.
"Navalny is now in a prison camp and his life is hanging by a thread. We don't know how much longer he can hold out," they said. "The life of Alexei Navalny and the fate of Russia depend on how many citizens take to the streets on Wednesday."
Navalny's daughter Dasha, a student at Stanford University, made an appeal on Twitter on Sunday for her father to see a doctor of his choice.