Navalny's wife Yulia said her husband now weighed 76kg (168 pounds), which is down 9kg since starting his hunger strike, according to AP. The placard with an image of the Kremlin critic reads "Freedom to Navalny!"
"A patient who has such a level of potassium should be monitored in intensive care, because he may develop a fatal case of arrhythmia at any time".
Prison authorities said Mr Navalny refused proper treatment and insisted he should be treated by a doctor of his choice from outside the facility, a request they have denied.
Doctors for jailed Russian opposition leader Alexei Navalny said he could die imminently unless he receives medical treatment.
Navalny was arrested on January 17 when he returned to Russian Federation from Germany, where he had spent five months recovering from Soviet nerve-agent poisoning that he blames on the Kremlin. "On the basis of having the poison and then on a hunger strike".
'This means both impaired renal function and that serious heart rhythm problems can happen any minute, ' said a statement on the Twitter account of Anastasia Vasilyeva, his personal doctor. She said union members hadn't examined Navalny but had received the tests from his lawyers, per Reuters.
FILE: Russian opposition leader Alexei Navalny gestures during a hearing on his charges for defamation in the Babuskinsky District Court in Moscow, Russia, Feb. 16, 2021.
Alexei Navalny, 44, almost died in August 2020 when he was poisoned with the chemical nerve agent, Novichock. "With his condition it's a matter of days".
Navalny has said prison authorities are threatening to put him in a straitjacket to force-feed him unless he abandons his hunger strike.
The open letter was published in Le Monde newspaper in France, The Economist, La Repubblica in Italy and Der Spiegel.
Navalny's team has earlier announced plans to stage what they said would be "modern Russia's biggest protest". As of Saturday, more than 440,000 people had signed up. "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.
Earlier this week, Navalny's wife Yulia, who visited him in the penal colony, said her husband now weighed 168 pounds - down almost 20 pounds since starting his hunger strike.