MOSCOW — In the lustrous, vaulted throne room of the czars who came before him, Vladimir V. Putin on Monday reclaimed the Russian presidency. A 30-gun salute cracked over the eerie quiet of the city, and Russia’s defense minister returned to Mr. Putin the black suitcase that contains the controls to a vast nuclear arsenal.

Outside the Kremlin walls, Mr. Putin announced his return in another way. The police swept boulevards and squares, detaining anyone they saw wearing white ribbons, the symbol adopted by anti-Putin activists.

Riot police officers in camouflage charged into cafes and restaurants in search of protesters, in one spot sending cups and glasses flying. Once in police custody, scores of young men were referred to military draft offices.

The clampdown underlined the challenge ahead for Mr. Putin, who even as the sweeps were taking place promised to expand Russians’ rights and freedoms, as well as their direct participation in government.