MOSCOW — Forcing a showdown with the government of President Vladimir Putin, leaders of the Russian political opposition took to the streets Saturday afternoon for an unsanctioned demonstration in a symbolic square in Moscow that is home to the headquarters of the federal security services as well as a monument to victims of Soviet political repression.

Bundled up against the cold and wind, with the temperature hovering at 5 degrees, about 2,000 demonstrators gathered in Lubyanka Square, not far from the Kremlin. They were met by a huge contingent of riot police, who quickly cordoned off the area and began making arrests.