Russia's leading opposition politician Alexei Navalny announced on Saturday he could now walk with a "tremble", and gave the first detailed account of his recovery nearly a month after being poisoned with Novichok nerve agent.

The 44-year-old Kremlin critic posted a photo of himself walking downstairs on Instagram and described how earlier symptoms had included the inability to form words.

"Now I am a guy whose legs tremble when he takes the stairs," he wrote, detailing moments of "despair" as doctors help him overcome the effects of the nerve agent.

This latest update on his progress came after posted to Instagram on Tuesday that he had spent a first day breathing unassisted, writing ironically: "It's an amazing process that's undervalued by many. I recommend it."

The anti-corruption campaigner fell ill on a plane from Siberia to Moscow on August 20 and spent two days in a Russian hospital before being airlifted to Berlin's Charite hospital.

Navalny said in his update that during the initial days of his recovery, he had needed therapy to help him recover his speech as he struggled to form words.

"Not long ago, I didn't recognise people and couldn't understand how to speak," he said.

"How to find a word and how to make it mean something? This was all totally incomprehensible.

"I didn't know how to express my despair either and so I was just silent."

The nerve agent Novichok disrupts communication between the brain, the main organs and muscles, while doctors say it gradually clears from the body.

Navalny, who said that he did not remember the early stage of his recovery, thanked the "fantastic doctors" treating him at Charite hospital.

He now saw a "clear path, although not a short one" to recovery, he said.

The message is characteristic of Navalny's fluent, ironic style of writing.

An avid user of social media, Navalny said he hoped soon to "become the highest form of life in modern society" and be "able to scroll through Instagram and add likes without thinking about it".

Suggesting he is struggling with fine motor skills, he said he was still unable to use a phone -- meaning friends or family probably posted the messages for him -- while "pouring myself a glass of water is a real performance".

Navalny has not assigned any blame in his messages so far but supporters and some European leaders have said that poisoning with Novichok, a military-grade nerve agent, points to a state-ordered crime.

The Kremlin has dismissed as "absurd" allegations it was behind the poisoning, saying it wants to know what happened.

Evidence of Novichok poisoning sparked calls for new sanctions against Russia and for Germany to abandon a near-completed project to carry Russian gas to Europe, Nord Stream 2.

Russia insists medical tests its doctors carried out found no poison in Navalny's body. It says it lacks grounds for a criminal investigation, despite international calls for a transparent probe.

Low-ranking Siberian transport police have carried out a basic examination of Navalny's movements.

Germany announced earlier this month that medical tests from a military chemical weapons laboratory had found "unequivocal evidence" of the nerve agent.

Navalny's aides said on Thursday that Germany had found traces of Novichok on a water bottle from the politician's hotel room in the Siberian city of Tomsk.

The Kremlin criticised their actions in removing evidence from the scene, while the aides said they did this because they expected Russia not to open an investigation.

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