MINSK, Belarus — The authoritarian government of Belarus blocked access to Facebook, Twitter and a major Russian social networking site Sunday in an attempt to prevent opposition protests on a national holiday. Thousands of police and special forces were deployed in the center of Minsk, the capital.

The respected rights group Vesna said the government also detained dozens of opposition activists, including Stanislav Shushkevich, Belarus’ first post-Soviet leader. Many other activists were called in by the KGB and warned not to protest, Vesna spokesman Valentin Stefanovich said.