Although he is 41, past the usual peak for chess players, Mr. Kasparov's announcement took many fans by surprise. He had just won his second elite tournament in a row and seemed to be playing with a vigor that he had not shown in some time. No one has retired while ranked as the world's top player since the American Paul Morphy did in 1859.

Mr. Kasparov said his decision had been building for some time. He said it was partly fueled by his frustration with the World Chess Federation because it had been unable to arrange a match between him and its current champion, Rustam Kasimdzhanov. But "I had no more goals in chess," he continued, adding, "I want to do things that excite me."

His priority now, he said, is to help drive President Vladimir V. Putin from power. "Democracy does not exist in Russia right now," he said emphatically.

Some criticism has followed predictable party lines. In a radio interview, Vladimir V. Zhirinovsky, the nationalist leader of the Liberal Democratic Party, dismissed Mr. Kasparov's move as his biggest mistake.

But even some like-minded politicians have raised doubts. Grigory A. Yavlinsky, leader of the Yabloko Party, suggested that Mr. Kasparov had yet to prove his political acumen. "Let him try to create a party, and succeed or fail," he told the magazine Itogi. "And we will talk to him after that."

The Kremlin, not surprisingly, has said nothing.