MOSCOW — A Moscow district court ordered Sergei Udaltsov, a prominent opposition leader, to be placed under house arrest on Saturday, in one of the most assertive legal measures to date against a leader of the anti-Kremlin protests that began more than a year ago.

Mr. Udaltsov, the leader of the radical socialist Left Front movement, faces a charge of conspiracy to incite mass disorder under a statute that can bring a maximum sentence of 10 years in prison. According to Saturday’s ruling, he may not leave his house, use the Internet, receive letters or communicate with anyone outside his family and legal team until April 6, the current date for the end of the investigation into his case.

The ruling seemed to signal a new stage in the government’s effort to punish well-known critics of President Vladimir V. Putin. Though most of the prominent protest leaders have served short sentences for administrative violations and several are the subject of criminal inquiries, none has yet been held or brought to trial on criminal charges.

The authorities may have held back from jailing protest leaders like Mr. Udaltsov for fear of inciting a backlash from opposition sympathizers. Mr. Udaltsov has a particular ability to mobilize young men and is one of the few opposition leaders to focus on economic issues relevant to Russians outside large cities. A passionate public speaker and the great-grandson of a prominent Bolshevik, Mr. Udaltsov stood out among the Moscow protesters, many of them middle-class Russians who distance themselves from calls for revolution.