Mr Navalny, a lawyer by training, earned a reputation as a Kremlin enemy writing about official corruption.
His activism expanded to organising anti-government protests and seeking political office, and over the years he had experienced frequent jailings, a chemical attack and an unexplained illness.
Now, his family, friends and supporters have a new reason to worry.
The 44-year-old opposition leader is in a grave condition in hospital after he became ill on a flight back to Moscow and fell into a coma.
His allies suspect he drank poisoned tea before boarding the plane.
Mr Navalny’s suffering is a shock and a worry to supporters who see him as a stalwart in Russia’s beleaguered opposition.
“Many times I was asked publicly and privately how I can support this terrible Navalny … I always answered the same way: Alexei Navalny risks his life every day for his beliefs,” Grigory Chkhartishvili, a dissident author noted for detective novels under the pen-name Boris Akunin, said on social media after Mr Navalny’s illness was announced.
Mr Navalny began his rise to prominence by focusing on corruption in Russia’s murky mix of politics and business.
In 2008, he bought shares in Russian oil and gas companies so he could push for transparency as an activist shareholder.
Mr Navalny’s work to expose corrupt elites had a pocketbook appeal to the Russian people’s widespread sense of being cheated.
Whether he was writing for his website or running for public office, his targets were likely to better resonate with potential supporters than more abstract goals such as democratic ideals and human rights.
Russia’s state-controlled television channels ignored Mr Navalny, but his investigations of dubious contracts and officials’ luxurious lifestyles got wide attention through the back channels of YouTube videos and social media posts.
The information uncovered by his Fund For Fighting Corruption mostly overrode the reservations that Mr Navalny’s nationalist streak and his advocacy for the rights of ethnic Russians raised, even in opposition circles.
Mr Navalny also understood the power of a pithy phrase and a potent image.
His description of President Vladimir Putin’s power-base United Russia party as “the party of crooks and thieves” attained instant popularity.
A lengthy investigation into then-prime minister Dmitry Medvedev’s lavish country getaway boiled down to the property’s well-appointed duck house; yellow duck toys soon became a way of deriding the prime minister.
The founder of two opposition political parties, he could also be flippant in the face of difficulty, tweeting sarcastic remarks from police custody or courtrooms on the many occasions he was arrested.
In 2017, after an assailant threw green-hued disinfectant in his face, seriously damaging one of his eyes, Mr Navalny joked in a video blog that people were comparing him to comic book character the Hulk.
Mr Navalny was frequently jailed for participating in protests – or sometimes even as he headed to them.
Online video reports of protests broadcast from Mr Navalny’s studios were sometimes enlivened by on-camera police raids.
He also faced more serious legal troubles.
In 2013, on the day after Mr Navalny had registered as a candidate for Moscow mayor, he was sentenced to five years in prison for an embezzlement conviction.
He was accused of stealing timber from a company in a region where he was an adviser to the reformist governor.
But in a hugely surprising move, the prosecutor’s office appealed against the sentence hours later.
The opposition attributed his release to the massive protests that greeted news of Mr Navalny’s imprisonment, but many observers thought it was a calculated move by authorities to make sure the mayoral election two months later carried a tint of legitimacy.
Mr Navalny ended up placing second, an impressive performance against the incumbent mayor with the backing of Mr Putin’s political machine and who was popular among Muscovites for improving the capital’s infrastructure and aesthetics.
The embezzlement conviction was eventually reinstated, and Mr Navalny was convicted, along with his brother Oleg, in another embezzlement case in 2014.
His brother received a three-and-a-half year prison sentence, while Mr Navalny’s sentence was suspended.
Although he did not get sent to prison, the conviction blocked Mr Navalny from being able to carry out his plans to run against Mr Putin in Russia’s 2018 presidential election.
His own legal obstacles and the widespread obstruction authorities set before other independent candidates seeking public office led Mr Navalny and his organisation to adopt a new strategy for the 2019 Moscow city council elections.
The Smart Vote initiative analysed which candidate in each district appeared to have the best chance of beating United Russia’s pick and tried to drum up support for that candidate.
The initiative appeared to be a success, with nearly half of the city council seats going to “systemic opposition” candidates, although its effectiveness could not be quantified.
Mr Navalny intended to redeploy the same strategy in next year’s national parliament elections.
But the Moscow city council races may have foretold even worse troubles for Mr Navalny.
While jailed last summer for taking part in a pre-election protest against the exclusion of many independent candidates, Mr Navalny became ill and was taken to hospital.
The official version was that he had suffered an allergic reaction.
His supporters and some doctors said at the time that poisoning appeared to be a more likely explanation.
