Mr. Kasparov had handed the bullhorn to Sergey V. Gulayev, a member of an opposition faction in the local legislature in St. Petersburg.

“The government is afraid of the slightest wind,” Mr. Gulayev he told the crowd. “The government is fragile, and afraid, and will collapse with one push.”

As he spoke, riot police shoved through the crowd and grabbed the bullhorn from his hands, smashing it against the wall of a building. A policeman put Mr. Gulayev, grimacing, in a headlock and dragged him into a police vehicle as members of the crowd yelled “Shame! Shame!”