A charismatic anti-corruption campaigner, Alexei Navalny has been Russia's leading opposition politician for around a decade, determined to challenge Vladimir Putin's grip on power despite frequent prison stays and even damage to his health.
The Yale-educated 44-year-old lawyer -- who was in intensive care in a Siberian hospital on Thursday after allies claimed he was poisoned -- has been banned from state television and was barred from challenging Putin in the 2018 presidential election.
He has often been jailed and physically attacked, but has vowed to press ahead with his efforts.
Navalny has won a young fan base through viral videos exposing corruption among the elites and has more than two million followers on Twitter.
He has grabbed attention with his uncompromising rhetoric and coined phrases such as the "party of crooks and thieves" to slam the ruling United Russia party.
In 2011, the anti-corruption blogger led mass protests when tens of thousands took to the streets of Moscow to protest against vote-rigging in parliamentary elections.
Two years later the father-of-two stood for Moscow mayor, coming second against Putin ally Sergei Sobyanin.
In 2017, he accused then-prime minister Dmitry Medvedev of massive corruption in a YouTube documentary. That kick-started a fresh wave of protests across the country that was met with police violence and mass arrests.
The same year he had to travel to Spain for surgery after one of several street attacks left him nearly blind in one eye.
Navalny has faced a series of legal cases, which supporters see as punishment for his activism.
In 2013, he was found guilty in an embezzlement case involving a timber deal and given a five-year suspended sentence that disqualified him from standing for public office.
In 2014, he was given another suspended sentence, and his brother Oleg, a co-defendant, was jailed for three-and-a-half years in a decision activists compared to a "hostage-taking".
Navalny has said he learnt about political campaigning from watching the US television series "House of Cards" and once listed Hollywood actor-turned-politician Arnold Schwarzenegger among his personal heroes.
Navalny toured Russia ahead of the 2018 presidential election in an American-style campaign to rally his supporters.
But with the Kremlin tightly controlling the media, he still remains a fringe figure for many Russians, who are exposed to the official portrayal of him as a Western stooge and convicted criminal.
Putin has refused to pronounce Navalny's name in public, instead referring to him as "the person you mentioned", among other euphemisms, when asked directly about the opposition leader.
While barred from mainstream politics, Navalny has kept trying to expose the lavish wealth of Russia's elites, broadcasting the findings of his investigations to millions of Russians on social media and YouTube.
Trawling through land registries and the filings of offshore companies, Navalny and his team have helped lay bare the mansions and hidden fortunes of high-ranking officials.
Among Navalny's most eye-catching exposes have been details on the palatial homes of Putin's allies in Russia and abroad -- including one kitted out with a vast climate-controlled storage room for fur coats.
But despite tapping into discontent among a largely young urban middle class he is far from a unifying opposition figure and some have criticised his anti-immigrant nationalist stance.
He scored his biggest recent success in local elections last year, when pro-Putin parties suffered losses because of a "Smart Voting" plan Navalny put forward after his allies were barred from standing in several races.
The tactic -- which called for voters to support the one opposition candidate most likely to defeat the ruling party -- saw Kremlin-backed lawmakers drop from 38 out of 45 seats in the Moscow city assembly to 25.
In the year since the elections, Navalny's offices have been raided repeatedly, while his Anti-Corruption Foundation was declared a "foreign agent" and ordered to pay several large fines.
Navalny was travelling the country to promote the tactical voting strategy in this September's regional elections when he fell ill and his plane made an emergency landing in the city of Omsk.