Alexei Navalny was jailed in February | GETTY IMAGES
Russian opposition leader Alexei Navalny has been moved to a prison hospital, where the authorities say his condition is "satisfactory".
He has been on hunger strike for 20 days, complaining of inadequate medical attention. He is now in Vladimir, a city 180km (112 miles) east of Moscow.
The FSIN prison service says he is being examined by a doctor daily and he has agreed to take vitamins.
Navalny was poisoned with a nerve agent last August and only narrowly survived.
After his transfer to intensive care in Berlin, toxicologists concluded that the poison was Russian weapons-grade Novichok.
Western governments blamed the Russian state for the attack, and Navalny blamed President Vladimir Putin directly. The Kremlin denied any involvement and disputed the conclusion about Novichok.
The prison service said doctors had decided to move Navalny on Monday to a hospital in Penal Colony No 3 in the city of Vladimir. He was transferred from the Pokrov penal colony, which is about 80km closer to Moscow.
Russia's Interior Ministry has now warned the population against holding any pro-Navalny street protests. There were thousands of arrests at such protests in January.
Navalny supporters have called for rallies on Wednesday - the day of a state-of-the-nation speech by President Putin.
"Any aggressive actions by participants in unauthorised public meetings, especially attempts to provoke clashes with law enforcement officials, will be regarded as a threat to public safety and immediately suppressed," the ministry said.
Navalny's doctors warned he would "die within the next few days" if not given urgent medical attention for acute back pain and leg numbness.
He started a hunger strike on 31 March in protest at not being able to see his own medical team. His doctors say recent blood test results indicate he could suffer kidney failure and go into cardiac arrest at any moment.
Alexei Navalny: what you need to know
Earlier, the US warned Russia there would be "consequences" if Navalny, 44 - the most prominent critic of President Putin - were to die in jail.
The UK, France, Germany and the European Union also expressed their concern over his treatment and demanded his release.
Navalny was jailed in February for old embezzlement charges, which he claims are politically motivated.
The European Court of Human Rights found violations of justice in the case against him.
Human rights campaigners say the jail where Alexei Navalny is being held is known for its especially harsh conditions | REUTERS
Four doctors, including his personal physician, Anastasia Vasilyeva, wrote to prison officials requesting to see Navalny urgently.
In the letter, which Dr Vasilyeva posted on Twitter, the experts said Navalny's potassium had reached "critical levels".
"This means both impaired renal function and that serious heart rhythm problems can happen any minute," it explained.
Having blood potassium levels higher than 6.0 mmol (millimoles) per litre usually requires immediate treatment. Navalny's blood test results, which were obtained by his lawyers, showed his were at 7.1, the letter said.
The doctors urged prison officials to let them examine Navalny immediately, "taking into account the blood tests and his recent poisoning".
In an interview with the BBC's Andrew Marr recorded on Friday but aired on Sunday, Russia's ambassador to Britain, Andrei Kelin, said Navalny was not in danger.
"Of course he will not be allowed to die in prison, but I can say that Mr Navalny behaves like a hooligan absolutely in trying to violate every rule that has been established," he said.
Ambassador Kelin added that Navalny, who is serving a two-and-a-half-year jail term, would be released earlier if "he will behave normally".
Last week Navalny's wife Yulia said her husband now weighed 76kg (168 pounds) - down 9kg since starting his hunger strike.
Alexander Polupan, who was one of the doctors to treat Navalny after the poisoning, posted an image of his blood test results, and said they were an "absolute indication" that he needed urgent medical care or he would "die within the next days".
Meanwhile, more than 70 well-known writers, artists and academics, signed a letter calling on President Putin to ensure adequate medical care for Navalny.
The letter was published in The Economist and France's Le Monde newspaper, and included the signatures of Hollywood actors Jude Law, Ralph Fiennes and Benedict Cumberbatch, Harry Potter author JK Rowling and director Ken Burns.
US President Joe Biden told reporters on Saturday that Navalny's medical treatment was "totally unfair and totally inappropriate". In March, US intelligence concluded that the Russian government was behind the nerve agent attack. The Biden administration imposed sanctions on senior Russian officials, and now Moscow is doing similar in retaliation.
Source: BBC News