Former world champion Garry Kasparov has announced that he is to retire from competitive chess.
Kasparov opposes Russian President Vladimir Putin
He made the surprise announcement after winning a prestigious tournament in Spain for the ninth time.
"I will continue to play chess because it is a lot of fun, but no longer on a professional level," he said.
The 41-year-old said he had made the decision because of the intense pressure which he had been under over recent years.
The chess grandmaster, a leading critic of Mr Putin, heads a group of top Russian liberals who have joined forces to keep Vladimir Putin from staying in the Kremlin after 2008.
His group, called Committee 2008: Free Choice, has criticised Mr Putin's control over Russia's parliament and the country's media, and what it calls the "flat-out falsification of the last election's results".
They have vowed to ensure a new president is elected in 2008.