Alexei Navalny, reported to have died on Friday by Russia's prison service, was the fiercest domestic opponent of Vladimir Putin, whom he accused of trying to kill him and of having him jailed on bogus charges and deprived of medical care. He was 47.
The Kremlin dismissed the allegations and cast Navalny as a Western puppet and common criminal guilty of the charges he was convicted of - fraud, contempt of court and extremism. The prison where he was serving said it was investigating his death.
Regarded as the Kremlin's personal political prisoner by many Western governments, supporters say the fiery opposition politician was systematically persecuted for challenging Putin, Russia's paramount leader for over two decades.
A former lawyer, Navalny was barred from running for president in 2018, made detailed corruption allegations against the Russian elite and accused President Putin and his allies of being autocrats who had led Russia into a disastrous war in Ukraine.
The Kremlin suggested Navalny was a CIA stooge and troublemaker out to topple the authorities and turn Moscow into a pliant U.S. vassal state. Putin never mentioned him by name.
Navalny and his followers - many of whom were under 30 - were outlawed as extremists after holding noisy anti-government protests that were dispersed with force and encouraging tactical voting to try to oust pro-Kremlin candidates.
In the West, many people saw him as a courageous and charismatic opposition politician ready to risk everything for a country he said he believed could one day become free and happy.
In Russia his movement counted around 700,000 people at one point, but state media did not mention him for many years and public opinion was divided.
He used YouTube to urge young voters to put their faith in what he called the "beautiful Russia of the future", but found it harder to connect with people outside big cities.
Married with two children, Navalny survived what Western doctors said was a nerve agent poisoning attempt on his life in 2020 on board a plane in Siberia.
He later phoned one of the men he said had tried to poison him, posing as someone else, and was told that state security agents had put poison in his underpants.
He said of Putin at the time: "However much he pretends to be a great geo-politician, he'll go down in history as a poisoner. There was Alexander the Liberator, Yaroslav the Wise, and Putin the Underwear Poisoner."
Putin denied the Russian state had tried to kill Navalny, saying it would have "finished the job" if it had really wanted to eliminate him.
Navalny's allies said he was planning to formulate a policy platform and create a pool of people ready to govern when the Putin era ended. He had urged his followers to turn out to vote in Russia's presidential election next month at noon to express their opposition to Putin, who is widely expected to win.
He was physically attacked inside Russia by pro-Kremlin activists several times and almost blinded in one assault.
Before he was jailed in 2021, he said he and his family were followed everywhere by Russia's intelligence services. In a Reuters interview in 2017 he shrugged off the risks.
"The security services are following us. They follow my children, my wife (Yulia) and me. Cars are constantly passing by. I don't even pay attention to it any more, but Yulia is really bothered by it."
'RUSSIA IS MY COUNTRY'
The son of an army officer, Alexei Anatolievich Navalny was born on June 4, 1976 and grew up mainly in Obninsk, about 100 km (62 miles) southwest of Moscow.
He obtained a law degree and a separate finance degree, and spent time in the United States on a fellowship at Yale, which pro-Kremlin critics seized on to suggest he was a foreign agent, a charge he and allies dismissed.
When protests erupted in Moscow following what critics said was the ruling United Russia's fraud-tainted election victory in December 2011, Navalny came to international prominence with his fiery speeches and was one of the first people arrested.
Internet-savvy and often dressed casually in jeans, he cut a stark contrast to the conservative image of former KGB officer Putin, whom he accused of "sucking the blood out of Russia."
His description of the ruling party as "crooks and thieves" resonated with followers and he was detained multiple times in the following years. In 2013 he was sentenced to a five year term on corruption charges before being freed the following day.
Later that year, he ran for mayor of Moscow, losing to a Kremlin-backed candidate but gaining 27% of the vote, a result his supporters cast as impressive given no state media coverage.
He unnerved some liberals in Russia's divided opposition with what they saw as a populist and nationalist streak, joining an annual nationalist march, calling for a crackdown on illegal immigration, and advocating a visa regime for Central Asians.
Fond of calling Putin "the old man in his bunker" during the COVID-19 pandemic, he promoted sophisticated investigations on YouTube into what he said was vast official corruption, a strategy he hoped would in time erode popular support for Putin.
His group used drones to film the luxurious residences of officials whose wealth they were scrutinising. He was repeatedly sued. One video alleging that Putin is the ultimate owner of an opulent palace, something the president denies, has been viewed more than 128 million times.
Navalny returned to Russia in 2021 after being treated in Germany for his poisoning, knowing he faced certain arrest.
"It was never a question of whether to return or not. Simply because I never left. I ended up in Germany after arriving in an intensive care box for one reason: they tried to kill me," Navalny wrote before his return.
"Russia is my country, Moscow is my city and I miss it."
In August 2023, Navalny was sentenced to an extra 19 years in prison on top of 11-1/2 years he was already serving in a criminal case that he said was designed to cow the Russian people into political submission.
The charges related to his role in his then-defunct movement inside Russia, which the authorities accused of trying to foment a revolution.
"I understand perfectly well that, like many political prisoners, I am serving a life sentence," he said at the time. "Where the life sentence is measured by the length of my life or the length of the life of this regime."