Putin critic Alexei Navalny has been pictured walking down stairs in hospital as he continues to recover after being poisoned with novichok.
He said in a post on Instagram that he still has difficulties climbing stairs because his legs tremble but his path to recovery is now "clear, although long".
On Thursday, a member of his entourage claimed he was poisoned via a water bottle in his Siberian hotel room.
Two independent laboratories in France and Sweden confirmed an earlier German finding that the Russian was poisoned with novichok, which was used in the Salisbury attack on ex-Russian spy Sergei Skripal.
Mr Navalny, the leading opponent of Russian President Vladimir Putin, fell ill in Siberia last month and was airlifted to Berlin where he has been treated in the Charite University Hospital.
German chancellor Angela Merkel said the use of the chemical weapon showed the "dangerous" attack on Mr Navalny was attempted murder and the aim was to silence him.
She said there were "very serious questions that only the Russian government can answer and must answer".
Mr Navalny said in the Instagram post he still could not use his phone and had difficulties trying to pour water into his glass.
"There are many problems yet to be solved but amazing doctors from the Charite hospital have solved the main one," the post said.
"They turned me from a 'technically alive human being' into someone who has high chances to become... a man who can quickly scroll Instagram and understands without thinking where to put his likes."
He added that at first he had had difficulties recognising people and finding the words to express himself, saying he suffered "despair" because he couldn't "find the words".
He wrote: "Although I understood in general what the doctor wanted, I did not understand where to get the words. In what part of the head do they appear in?
"I also did not know how to express my despair and, therefore, simply kept silent.
"Now I'm a guy whose legs are shaking when he walks up the stairs, but he thinks: 'Oh, this is a staircase! They go up it. Perhaps we should look for an elevator,'
"And before, I would have just stood there and stared."
Hamish de Bretton-Gordon, the former head of the British army's chemical weapons unit, said he was certain that lessons learned from the attack in Salisbury had been employed in treating Mr Navalny.
He told Sky News: "We learned an awful lot about novichok after the Salisbury poisonings back in March 2018 and we are fortunate in this country to have the most brilliant scientists at Porton Down who worked out exactly what it was and with the National Health Service how to treat the casualties.
"Sadly, Dawn Sturgess died, but five survived and I understand the medical information gained from that was shared with the Germans and was no doubt key to the survival of Alexei Navalny."
Mr de Bretton-Gordon called on the Kremlin to allow the Organisation for the Prohibition of Chemical Weapons in to investigate the use of novichok on its soil and warned that a clean-up would be needed to prevent anyone else being affected.
He added: "The Russians are signatories to that. Novichok is a proscribed illegal chemical weapon and it has been used on Russian soil and the Russians should get them in to get to the bottom of this, because if they are not guilty, the OPCW will find out.
"Because of the way the novichok was delivered to Navalny in a bottle, we know that a quarter of an egg cup of novichok on a door handle in Salisbury spread all over Salisbury and took 18 months to clean up...
"This is probably the same... and I would hope the Russians would be working hard in Siberia and elsewhere to find out where this contamination is, otherwise there could be further casualties."
The Kremlin has repeatedly stated that until Mr Navalny was transferred to Berlin, Russian labs and a hospital in the Siberian city of Omsk found no sign of novichok poisoning.
Moscow has called for Germany to provide evidence of any use of novichok or foul play.
Kremlin spokesman Dmitry Peskov said Friday: "There is too much absurdity in this case to take anyone at their word."