MOSCOW — Opposition leader Sergei Udaltsov was charged Friday with plotting mass disorder, part of a clampdown by Russian authorities on public dissent, but was unexpectedly allowed to remain free on his own recognizance.

The decision to release him surprised Udaltsov, a 35-year-old socialist, and his attorney, who had predicted Udaltsov would join two associates in jail. One supporter suggested that the authorities were hoping Udaltsov would flee the country, which would discredit him and deprive his Left Front bloc of its leadership. If convicted of plotting mass disorder, he would face up to 10 years in prison.