MINSK, Belarus – A Belarusian opposition leader was arrested yesterday and sentenced to 15 days in jail for taking part in an unsanctioned protest rally.

Police detained Alexander Milinkevich yesterday, a day after a rally in Minsk against authoritarian President Lukashenko.

The rally had received official approval, but police declared a march to the rally site to be unsanctioned.

Mr. Milinkevich said after his sentencing he initially thought he was being charged in connection with that march. But the court told him police had declared the rally itself illegal, he said.

“I came to the meeting … considering it to have been sanctioned. Police there did not say it had not been sanctioned. Therefore, I think the charge is not just and I do not consider myself guilty,” he said.

“This is a political sentence. The leaders of political parties are being put behind bars,” Mr. Milinkevich said.

The jailing appeared to sharply escalate authorities’ attempts to repress the opposition. Although an array of opposition figures has been arrested in recent weeks, most of them do not have the wide visibility of Mr. Milinkevich.

In recent weeks, Mr. Milinkevich has made several trips to Western countries to seek support for the opposition.

Mr. Milinkevich spearheaded an unprecedented week of protests after the March 19 presidential elections in which Mr. Lukashenko won a third term with more than 80% of the vote, according to official results that the opposition alleges were fraudulent.

Those protests ended when police violently dispersed a crowd trying to march to a jail to demand the release of protesters who had been arrested on the central square in Minsk.

Another opposition presidential candidate, Alexander Kozulin, was arrested in that clash and remains in jail.