Alexei Navalny is the most vocal and effective critic of Russia's government and President Vladimir Putin. He is in pre-trial detention after returning to Russia from Berlin, where he was treated for the near-fatal effects of a military-grade nerve toxin that he had been given while travelling within Russia.
Here are some details about him:
How did he become known?
Navalny, 44, is best known in Russia for producing detailed investigations into corruption, which get millions of views on his YouTube channel. His name for the ruling United Russia party - the "party of crooks and thieves" - has become common coinage among his supporters.
Calls for restrictions on immigration and criticism over what some viewed as his overly nationalist views prompted his expulsion from the liberal Yabloko opposition party in 2007.
When demonstrations against Putin flared in December 2011, after an election tainted by fraud, he was one of the first protest leaders arrested.
Has he run for office?
Having emerged as one of the figureheads of a divided opposition, he ran for mayor of Moscow in 2013. He lost to the Kremlin-backed Sergei Sobyanin, but received 27% of the vote despite getting little or no coverage on the state-controlled main media outlets.
Barred since then from running for office on various grounds, which he says are trumped up by the Kremlin, he uses social media and personal charisma to mobilise crowds of mostly young protesters. Putin and the Kremlin make a point of never saying his name.
What are his legal troubles?
Navalny has been detained countless times for organising public rallies, and prosecuted repeatedly on charges that he says are politically motivated.
In 2013, he was sentenced to five years for corruption, although an appeal resulted in the sentence being suspended. In 2014, he was given a suspended three-and-a-half-year sentence for embezzlement, the terms of which he is now accused of violating.
How has he been treated?
In 2016, Navalny and his supporters were physically attacked by several men while on a trip to southern Russia. In 2017, he was attacked twice, and a green antiseptic dye was thrown at him, causing him temporarily to lose sight in one eye.
Last August, Navalny fell ill on a flight from Tomsk, in Siberia. The pilot made an emergency landing, saving his life, and Navalny was flown to Berlin, where he was treated for the effects of a neurotoxin that German military tests showed to be Novichok, a poison developed in the Soviet Union.
A joint media investigation said it had identified a team of assassins from Russia's FSB security service. Putin dismissed the investigation as a smear, saying: "If someone had wanted to poison him, they would have finished him off."
What is his background?
The son of an army officer, he was born on June 4, 1976, and grew up mainly in Obninsk, about 100 km (60 miles) southwest of Moscow. He has a degree in law, and spent time in the United States on a fellowship at Yale University.
He also has a degree in finance, and bought small stakes in some of Russia's biggest companies to get access to their financial statements and demand greater transparency.
He is married to Yulia Navalnaya and has a daughter and a son.