MOSCOW — Anti-government protesters managed to gather a third huge crowd in the center of Moscow on Saturday, undeterred by arctic, bitter cold or by the near certainty that Vladimir V. Putin will win a six-year presidential term next month.

This time, Russian authorities were prepared, organizing a simultaneous, and also huge, rally in defense of Mr. Putin. Speakers there issued dire warnings of the possible consequences of continued protest: revolution and the breakup of the country.

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