Allies of Alexei Navalny have called on his supporters to stage mass protests on Wednesday in towns and cities all across Russia, amid a dire warning that the jailed Kremlin critic and opposition leader is now dangerously ill and could die “at any minute”. Navalny’s team said the situation had got so desperate that there was no time to delay. They had previously said street protests would resume once they reached 500,000 signatures in support – with the current tally about 50,000 short. In a video posted on Navalny’s YouTube channel his deputies Leonid Volkov and Ivan Zhdanov said Navalny’s health had deteriorated so dramatically that a mass public display was the only way to save him. Volkov urged citizens to gather at 7pm on Wednesday in squares across the country. The appeal sets up a showdown between Navalny’s followers and Vladimir Putin, who is due to deliver his annual state of the nation address at the same time. In January the Kremlin used brutal force to break up pro-Navalny street protests, with thousands of people arrested. “Have you ever seen with your own eyes how a person is murdered? You’re seeing it right now,” Volkov said. He added: “If we don’t speak up now, the darkest times for free people are at hand. Russia will descend into total hopelessness.” In a further clampdown, Russia’s prosecutor’s office is set to designate Navalny’s FBK anti-corruption foundation and his regional headquarters as extremist organisations. This would allow the authorities to jail Navalny’s colleagues as “terrorists” for up to six years. Over the weekend Navalny’s allies said he risked cardiac arrest at “any minute” and could be dead in a matter of days. He has been on hunger strike for almost three weeks and has demanded – without success – that an independent medical team be allowed to examine him. On Sunday Navalny’s daughter Dasha pleaded on Twitter for her father to be given the care he needs. “Allow a doctor to see my dad,” she wrote. Navalny returned to Moscow in January from Germany after recovering from an assassination attempt. A secret unit from Russia’s FSB spy agency poisoned him last summer with the nerve agent novichok while he was on a trip to Siberia, he alleges. The Kremlin denies the claim. Navalny was immediately arrested. He was then convicted in a case he and western governments say is politically motivated. Navalny has recently complained of losing feeling in his legs and arms. His wife, Yulia, who visited him last week said he now weighed 76kg – down 9kg since starting his hunger strike – and was so weak he had to lie down. There are few signs the Kremlin is willing to relent or give in to demands by the US president, Joe Biden, and other western leaders for Navalny to be released from custody. On Sunday Russia’s ambassador to the UK, Andrei Kelin, accused Navalny of attention-seeking behaviour. “He will not be allowed to die in prison, but I can say that Mr Navalny, he behaves like a hooligan, absolutely,” Kelin told the BBC. The ambassador added: “His purpose for all of that is to attract attention for him[self].” 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 about 60 miles (100 km) east of Moscow. Navalny’s personal doctor, Anastasia Vasilyeva, and three more doctors, including cardiologist Yaroslav Ashikhmin, have asked prison officials to grant them immediate access. “Our patient can die any minute,” Ashikhmin said on Facebook on Saturday, pointing to the opposition politician’s high potassium levels and saying Navalny should be moved to intensive care. “Fatal arrhythmia can develop any minute.” 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. “This means both impaired renal function and that serious heart rhythm problems can happen any minute,” said a statement on Vasilyeva’s Twitter account. The doctors said he had to be examined immediately “taking into account the blood tests and his recent poisoning”. Navalny’s spokesperson, Kira Yarmysh, who accompanied him when he collapsed on an internal flight after the poisoning in August, said the situation was critical again. “Alexei is dying,” she said on Facebook. “With his condition, it’s a matter of days.” She said she felt like she was “on that plane again, only this time it’s landing in slow motion”, pointing out that access to Navalny was restricted and few Russians were aware of what was actually going on with him in prison.