Tens of thousands of people gathered in the capital, Minsk, for the protest, which international observers and independent journalists said was largely peaceful until helmeted riot police officers wielding clubs and shields dispersed it. The government, however, accused Mr. Sannikov and other opposition leaders of fueling mass unrest that led to the breaking down of doors of a government building in Minsk.

The resulting crackdown, observers say, has been the most severe of Mr. Lukashenko’s nearly 17-year rule. Seven of the nine opposition candidates were among the hundreds arrested after the protest, and over two dozen opposition figures have been convicted and sentenced to up to four years in prison.

Mr. Sannikov has spent the last five months in a cell at the prison of the K.G.B. in Minsk.

In testimony, Mr. Sannikov accused prison guards of torturing him and other opposition figures, depriving them of sleep, exposing them to severe cold and threatening family members in an effort to secure confessions.

Mr. Sannikov accused the head of the K.G.B., Vadim Zaitsev, of threatening to harm his wife and young son if he did not agree to give incriminating testimony.

“When I refused to affirm this lie that was offered to me, when I refused to write a petition, Zaitsev literally said the following: ‘Then harsh measures will be taken against your wife and child,’ ” Mr. Sannikov said, according to a transcript of his testimony published to the opposition Web site, Charter 97.