By C. J. CHIVERS
The announcement, during which prosecutors also said the police had arrested more than 500 demonstrators last week, continued the crackdown on the opposition to President Aleksandr G. Lukashenko, who is often called Europe's last dictator.
It was also an unmistakable snub to the West. The United States and the European Union have demanded the release of the candidate, Aleksandr V. Kazulin, and of demonstrators arrested last week during peaceful protests against Mr. Lukashenko's re-election, which the opposition says was a fraud.
Mr. Kazulin, who was arrested Saturday as he led a march toward a detention center where opposition members were being held, faces charges of organizing group actions and hooliganism, Belarus's federal prosecutor said, according to the Belapan news agency. The first charge could carry a prison term of up to six years.
Mr. Kazulin received 2.2 percent of the officially tabulated vote on March 19.
In an interview Friday, he said an honest count would show he received roughly a third of the votes cast. The next day, he led a group of demonstrators who were attacked by an elite riot police unit, SOBR, which has been widely accused of human rights abuses.
His wife, Irina Kazulin, said the charges were retaliation for his daring to challenge Mr. Lukashenko's authoritarian rule. "It is personal revenge," she said by telephone. "It is purely a political case."
Ms. Kazulin also said her husband had been kicked and beaten, and had a back injury that had not been examined by a doctor. The prosecutor said Mr. Kazulin had not filed a complaint. Neither claim could be independently verified.
Prosecutors also said they were considering charges against Aleksandr Milinkevich, who received 6.1 percent in the official vote count and who has called for peaceful acts of public assembly. The potential charge, disturbing public order, could carry a sentence of 15 days.
Mr. Milinkevich, in a telephone interview as he prepared for meetings in Poland, said that he expected to be jailed, and that the announcement on Wednesday was part of a mounting campaign against him. "At first, it is usually a P.R. action," he said. "It is only beginning."
He added that the government's pressure was counterproductive.
"I think they would very much like me to emigrate, fearing imprisonment, but I am not going to do that," he said. "The more repression they bring against the protesters and the organizers of protests, the more protests they will have. They are losing their supporters."
The United States warned Belarus's prosecutors not to proceed with charges.
In a telephone interview, David J. Kramer, the deputy assistant secretary of state for European and Eurasian affairs, said officials involved in these cases risked personal sanctions in the West.
The European Union is expected to discuss sanctions against Belarussian officials when foreign ministers of its 25 nations meet on April 10. The United States, which has its own process for applying sanctions, including bans on travel and asset freezes, has been adding names of Belarussian officials on an almost daily basis since last week, Mr. Kramer said.