LONDON — There comes a moment in the life of every dictator when the mask of respectability slips and he reveals the ugly face of tyranny. Hitherto President Putin has been careful to preserve a semblance of legality, even as he set about hollowing out democracy, civil society, and the market economy in Russia. For without legality, there can be no legitimacy.

Over the weekend, however, with the brutal suppression of demonstrations in Moscow and St. Petersburg by the main opposition coalition, known as the Other Russia, and the arrest of its unofficial leader, Garry Kasparov, the Putin regime took a huge and irreversible step toward dictatorship. Why is the crushing of a single peaceful rally, at which heavily armed riot police far outnumbered the protesters, so significant? It may be a manifestation of fear on the part of the authorities: “People power” has brought down so many authoritarian governments since the miraculous year of 1989 that Mr. Putin would be foolish not to take the threat to his own system seriously.

Yet the Russian president professes to have no worries about the Other Russia, which does indeed embrace a spectrum of opinion that ranges from the National Bolsheviks on the far right to extreme leftists and disgruntled ex-ministers.

If the regime were really as confident of popular support as its leader pretends, however, it would not need to resort to legal chicanery and violence against the opposition. If the opinion polls purporting to show that Russians like the harsh policies pursued by the Kremlin are reliable, why does the self-styled savior of Mother Russia not simply ignore the Other Russia?

What has changed is that the opposition now has a brave and articulate leader. When it comes to charisma, Mr. Kasparov is in a different league from the Yeltsin-era has-beens and Putin-era cast-offs with whom the Kremlin has had to contend.

In fact, Mr. Kasparov’s public persona compares rather favorably with that of Mr. Putin himself. Despite a huge propaganda campaign designed to insinuate that a former world chess champion cannot be taken seriously as a potential leader, a growing number of Russians are impressed by what they have been allowed to see of Mr. Kasparov in action. They like what they hear or read about his detailed critique of Mr. Putin’s domestic and foreign policies.

Cynics here in the West who sneer that a former grandmaster of chess cannot possibly be a match for a former colonel of the KGB evidently don’t realize what a bear pit Mr. Kasparov inhabited during the 1980s. In those days, he had to fight the communist bureaucracy every inch of the way to get to the top in the one field in which the Soviet Union reigned supreme.

It is easy to forget how few people even in November 1989 believed that a playwright, Vaclav Havel, could humble the cruel old men who had invited the Red Army to crush the Prague Spring.

We know that Mr. Putin is ruthless in eliminating anybody who poses even a remote threat. The assassination of Anna Politkovskaya and the unsolved murders of many other journalists, not to mention the sensational polonium-210 poisoning of Alexander Litvinenko in London, have forced Mr. Kasparov to take reasonable precautions.

Until his arrest on Saturday, there had been an unspoken assumption that Mr. Kasparov was too much of a celebrity to be directly threatened by the shadowy organizations the regime uses to do its dirty work.

That assumption was never shared by those who know the methods in which Lieutenant Colonel Putin was trained. A former dissident who spent 10 years in the gulag, Natan Sharansky, is in regular contact with Mr. Kasparov. On a recent visit to London, Mr. Sharansky told me: “Garry’s life is in danger.”

Just as Mr. Sharansky tried to do in the worst days of the Brezhnev era, Mr. Kasparov has led his life as if Russia really were a democracy under the rule of law. The Kremlin has tried to pretend that its manipulation of the parliamentary and legal systems was justified by the oligarchs’ plundering of the state during the immediate post-Soviet period.

Now Mr. Putin’s pretense seems to have been abandoned. After his release from custody yesterday, Mr Kasparov said: Russia “is no longer a country … where the government tries to pretend it is playing by the letter and spirit of the law.”

We may be sure that Mr. Kasparov will not let himself be intimidated by the fact that the Russian secret police are now shamelessly going public with their tactics. He believes that the stability the Kremlin claims for itself to gullible members of the Western press has now been exposed as a sham.

Attempts will be made to link Mr. Kasparov and the Other Russia movement to Boris Berezovsky, the exiled former oligarch who is fighting extradition to Russia from Britain. Mr. Berezovsky was reported by the left-wing Guardian newspaper last week to have stated that the Putin regime could be removed only by force.

However, Mr. Kasparov is not an advocate of violence, merely of the right to peaceful protest. “The street is the only place where people can really express their views,” he says.

It would be a mistake to expect a repetition of Ukraine’s Orange Revolution on the streets of Moscow quite yet. But as the Putin presidency becomes ever more aggressive abroad, however, at home it seems increasingly paranoid. And the more paranoid a dictator becomes, the bigger the mistakes he tends to make. Those who lose their grip on reality soon lose their hold on power. For the first time since he came to office, Mr. Putin looks as if he too could be losing his grip.