“We never saw such a protest,” he said. “Everybody recognizes it is a new page.”

Mr. Kasparov, 43, is not Mr. Putin’s only critic, but he may be the most prominent. And he has brought to oppositional politics the same energy and aggression that characterized his chess, attacking Mr. Putin and the Kremlin — or the regime, as he repeatedly calls it — with language rarely spoken so bluntly in Russia.

“This regime is getting out of touch with the real world,” he said. “It’s a deadly combination of money, power and blood — and impunity.”

Such attacks have drawn the scrutiny of the authorities, though so far nothing worse; someone who sounded angry that Mr. Kasparov had given up chess for politics attacked him with a chessboard in 2005. (“I am lucky,” he said at the time, “that the popular sport in the Soviet Union was chess and not baseball.”)

He now travels with bodyguards. He hired them out of concern for hooligans, he said, not because other Kremlin critics have been killed, like the journalist Anna Politkovskaya, who was shot to death in Moscow last October.

“If the state goes after you,” he said, “there’s no stopping them.”

THIS is not the place Mr. Kasparov expected to be when he resigned from the world of professional chess two years ago, quitting while still the highest-ranked player, if no longer the world champion. He is a famous man and a wealthy one, the author of numerous books on chess and its lessons for life, who is now leading acts of civil disobedience in an uphill battle to protest Mr. Putin’s policies.