Alexei Navalny's teenage daughter has pleaded for doctors to be allowed access to her father, as allies of the jailed opposition leader called for mass protests in response to medical warnings that he could die "at any minute".
Several doctors, including Navalny's personal physician, warned at the weekend that the Kremlin's most prominent critic was in a critical condition because of "catastrophically high" potassium levels that might lead to heart failure.
"Allow a doctor to see my dad," Daria Navalnaya demanded yesterday on Twitter.
The 19-year-old, who, like her father, has a large social media following, is an undergraduate in psychology at Stanford University in California. Two years ago, she worked with the New York Times to produce a video on young Russians "frustrated with the status quo [and] pessimistic about the possibility of change".
Navalny, 44, has been on hunger strike since the end of last month, in protest over a lack of treatment for acute back pain that has led to numbness in his limbs.
He was arrested on his return to Russia at the start of the year, after recovering in Germany from a near-fatal nerve agent poisoning, and swiftly sentenced to two years in prison on charges relating to an embezzlement case dating back to 2014.
The opposition organised anti-Kremlin rallies in response, sparking a crackdown from police and thousands of detentions, before deciding to pause the street protest movement.
"Right now he is being killed in a prison colony. We can't wait any longer," Navalny ally Leonid Volkov said as he announced further demonstrations in all major Russian cities for Wednesday evening, shortly after President Vladimir Putin gives his annual state of the nation address.
More than 450,000 people have already registered to take part in the events. While the number is shy of a previously announced target of half a million, the figure suggests that Wednesday could see the country's biggest protests since the collapse of the Soviet Union three decades ago.
"Have you seen a person being killed with your own eyes? You have. You are seeing it now," Volkov said in a video released via the opposition leader's YouTube channel yesterday. "No matter how much you want to escape that thought, to change the topic - that does not change the fact that Alexei Navalny is being killed. Using terrible means. In front of all of us."
Doctors said that the levels of potassium in Navalny's blood were significantly higher than the level that usually requires medical intervention.
"This means both impaired renal function and that serious heart problems can develop at any minute," his personal doctor Anastasia Vasilyeva said in a statement.
Navalny has tried to keep an upbeat tone in messages passed to his lawyers and posted on social media, in which he has joked about his routine in prison. But he has also told of how prison guards wake him once an hour through the night, and of threats to put him in a straightjacket and force-feed him if he does not end his hunger strike.
The EU said it would discuss Navalny's case at a meeting of foreign ministers today, as French President Emmanuel Macron said it was time to draw "clear red lines" in relation to Russia, a shift away from his usual conciliatory tone.
Both the EU and the US have sanctioned Moscow over its treatment of the opposition leader.
Navalny has long accused the Kremlin of orchestrating his poisoning.
Last week, prosecutors moved to label his organisation an "extremist group", meaning supporters could face lengthy jail terms.
The EU said it was "deeply concerned" about Navalny's failing health yesterday, while the US warned Russia of "consequences" if he dies.