Belarus ? Belarus’ authoritarian government jailed the country’s most prominent opposition leader Thursday after a protest rally that attracted thousands, a move that drew swift international condemnation.

The sentences imposed on Alexander Milinkevich and three of his deputies threw into question the ability of activists to gather a crowd at a demonstration planned for Monday in their campaign to drive President Alexander Lukashenko from power.

Milinkevich, who has spearheaded the wave of unprecedented protests since Lukashenko’s re-election last month, was sentenced to 15 days in jail. Other opposition leaders received similar sentences in connection with Wednesday evening’s rally, which drew about 10,000 people.

The United States, the European Union, NATO and the 55-nation Organization for Security and Cooperation in Europe condemned Milinkevich’s jailing as a violation of democratic values and free speech. They demanded his release.