Former world chess champion Garry Kasparov, who was arrested at a banned opposition rally in Moscow over the weekend, has long been seen as a thorn in the side of the Russian establishment.
Those who have met Mr Kasparov are struck by his energy
When he first appeared on the world stage in the 1980s, he quickly became known for his defiance of the Soviet authorities and - while dominating at the chess board - has been a leading figure in Russia's reformist movement since the collapse of the USSR.
So great is Mr Kasparov's passion for politics that in 2005 he retired from chess to focus his efforts on defeating President Vladimir Putin.
Since then he has assembled a bewilderingly broad coalition, The Other Russia, which includes both mainstream politicians like the out-of-favour former Prime Minister Mikhail Kasyanov and fringe groups from both ends of the political spectrum.
But correspondents say the movement does not seem to enjoy genuine popular support.
Garry Kasparov was born in 1963 in Baku, in the then Soviet republic of Azerbaijan, to a father of Jewish descent and an Armenian mother.
He became the youngest ever world chess champion in 1985, defeating fellow Soviet Anatoly Karpov, and went on to dominate the game for the next two decades.
Many regard him as the greatest player the game has produced.
Described by colleagues as a complex character, both dynamic and highly ambitious, he has frequently been outspoken in the chess world as well as in politics.
In 1993, following a dispute with world chess body FIDE, he and British grand master Nigel Short set up a rival organisation. The chess world was split for several years.
He was generally considered to be the world number one until his defeat to fellow Russian Vladimir Kramnik in 2000, although there was no undisputed champion in those years.
Opponents have spoken of his physical presence at the chessboard, and all who meet him are struck by his energy and intensity.
His frustrations with world chess were a major factor in his retirement two years ago, when he admitted he no longer had the same passion for the game.
But his decision was also influenced by events in Russia, which he believed had been creeping towards authoritarianism since Mr Putin took charge, and in Ukraine, which had just undergone the Orange Revolution.
Since then he has surprised observers with his ability to hold together a seemingly unfeasibly disparate movement whose one common goal is the defeat of the current government.
The Other Russia combines elements of the now weakened and fragmented democratic movement with some of their bitterest former enemies - like the National Bolshevik Party, famous for its audacious anti-government stunts and quasi-Nazi symbols, or the far-left Workers' Russia, led by old-style communist firebrand Viktor Anpilov.
The movement's numbers are small, with recent rallies drawing only a few thousand people and generally outnumbered by police.
But correspondents say Mr Kasparov's high international profile is enough to persuade the Kremlin to keep a close eye on his moves.