MOSCOW — Tens of thousands of Russians took to the streets in Moscow on Saturday shouting “Putin is a thief” and “Russia without Putin,” forcing the Kremlin to confront a level of public discontent that has not been seen here since Vladimir V. Putin first became president 12 years ago.

The crowd overflowed from a central city square, forcing stragglers to climb trees or watch from the opposite riverbank. “We exist!” they chanted. “We exist!”

The demonstration marked what opposition leaders hope will be a watershed moment, ending years of quiet acceptance of the political consolidation Mr. Putin introduced. The leaders understood that for a moment they, not the Kremlin, were dictating the political agenda, and seemed intent on leveraging it, promising to gather an even larger crowd again on Dec. 24.

Saturday’s rally served to build their confidence as it united liberals, nationalists and Communists. The event was too large to be edited out of the evening news, which does not ordinarily report on criticism of Mr. Putin And it was accompanied by dozens of smaller rallies across Russia’s nine time zones, with a crowd of 3,000 reported in Tomsk, and 7,000 in St. Petersburg, the police said.