8-9 minutes - Source: BBC
He has called Mr Putin's party a place of "crooks and thieves", accused the president's system of "sucking the blood out of Russia" and vowed to destroy the "feudal state" being built.
He has led nationwide protests against the authorities.
But he has not been able to fulfil what is, perhaps, his biggest dream: challenge Mr Putin in the ballot box.
His candidacy in the 2018 presidential election was banned by authorities over his conviction by a Russian court of embezzlement, which bars him from running for office.
Mr Navalny vehemently denies the accusations, saying his legal troubles are Kremlin reprisals for his fierce criticism.
Many say it is surprising that he is still walking free.
However, on 20 August 2020 he was admitted to hospital with suspected poisoning. It is the second suspected poisoning for the opposition leader.
Last year, he was diagnosed with "contact dermatitis" whilst in jail, with his doctor suggesting he might have been exposed to "some toxic agent".
The anti-corruption campaigner fell ill during a flight and the plane made an emergency landing in Omsk, fellow campaigner Kira Yarmysh said on 20 August, adding that they suspected something had been mixed into his tea.
Prominent bloggerWhile he is a thorn in the Kremlin's side, Mr Navalny also has critics among opposition groups, not least for what some see as his Russian nationalism.
In 2014 he was asked about President Putin's annexation of Ukraine's Crimea peninsula, in an interview with Ekho Moskvy radio station. He said that despite Crimea being "seized" in violation of international law, "the reality is that Crimea is now part of Russia". "Crimea is ours," he said.
His rise as a force in Russian politics began in 2008 when he started blogging about alleged malpractice and corruption at some of Russia's big state-controlled corporations.
One of his tactics was to become a minority shareholder in major oil companies, banks and ministries, and to ask awkward questions about holes in state finances.
His use of social media to deliver his message symbolises his political style, reaching out to predominantly young followers in sharp, punchy language, mocking the establishment loyal to President Putin, who refuses to mention his name.
'Crooks and thieves'The campaign against corruption took Mr Navalny from criticism of corporations directly to opposition to the ruling party, United Russia.
Ahead of the 2011 parliamentary election, which he did not fight as a candidate, he urged his blog readers to vote for any party except United Russia, which he dubbed the "party of crooks and thieves". The phrase stuck.
United Russia won the election, but with a much-reduced majority, and its victory was tarnished by widespread allegations of vote-rigging that prompted protests in Moscow and some other major cities.
Alexei Navalny: The basics
- Born 4 June 1976 at Butyn, in the Moscow region
- Graduated in law at Moscow's Friendship of the Peoples University in 1998
- Became a Yale World Fellow in 2010
- Lives in Moscow with his wife and two children
Mr Navalny was arrested and imprisoned for 15 days following the first protest on 5 December 2011, but emerged to speak at the biggest of the post-election rallies in Moscow on 24 December, attended by as many as 120,000 people.
Mr Putin later won re-election as president easily and Russia's powerful Investigative Committee launched criminal investigations into Mr Navalny's past activities, even questioning his credentials as a lawyer.
When he was briefly jailed in July 2013 for embezzlement in the city of Kirov, the five-year sentence was widely seen as political.
He was unexpectedly allowed out of prison to campaign for the Moscow mayoral elections, in which he was runner-up with 27% of the vote, behind Putin ally Sergei Sobyanin.
That was considered a dramatic success as he had no access to state TV, relying only on the internet and word of mouth.
His conviction was eventually overturned by the Russian Supreme Court following a judgment by the European Court of Human Rights that he was not given a fair hearing at the first trial.
Then, in a retrial in 2017, he was convicted for a second time and handed a five year suspended sentenced. He called the judgment farcical, saying it was all an attempt to bar him from the 2018 election.
Fighting on, in RussiaAlthough Mr Navalny never had the public profile of former jailed oligarch Mikhail Khodorkovsky, parallels between the two figures have been drawn.
Mr Khodorkovsky spent a decade in Russian jails, and when in 2010 a court convicted him for a second time, the lengthy prison sentence was announced on 30 December, when most Russians were focused on the New Year holiday.
Unlike Mr Khodorkovsky, now based in Switzerland, Mr Navalny has vowed to fight on in Russia - he was arrested three times in 2017 for organising unauthorised anti-Putin protests and briefly detained in early 2018.
Speaking to the BBC, he suggested the best thing Western states could do for justice in Russia was to crack down on "dirty money".
"I want people involved in corruption and persecution of activists to be barred from entering these countries, to be denied visas."
When Mr Navalny was jailed in 2013, he told the judge that he would fight on with his colleagues "to destroy the feudal state that's being built in Russia, destroy the system of government where 83% of national wealth is owned by a half per cent of the population''.
Facing critics elsewhereMr Navalny has spoken at ultra-nationalist events, causing concern among liberals. Russian nationalists, too, were wary of his links with the US after he spent a semester at Yale in 2010.
But when the opposition elected its own leaders in October 2012, it was Alexei Navalny who won, ahead of veteran Putin critic and former chess champion Garry Kasparov, although it was on a small turnout of 81,801.
Russia's annexation of Crimea from Ukraine and its support for pro-Russian separatists in eastern Ukraine has been a problem for the anti-Putin opposition. Opinion polls suggest strong support for the intervention among Russians - the Kremlin denies fomenting the Ukraine conflict.
Mr Navalny has been an advocate of sanctions against Putin allies. But one question that has regularly been posed about Mr Navalny is whether he commands any support beyond the population centres of Moscow and other cities.