Authorities in Belarus have intensified their roundup of opposition activists ahead of Sunday's presidential elections.

Backers of the main opposition candidate, Alexander Milinkevich, say authorities have detained at least 50 campaign workers on various charges, and have sentenced some to terms of up to 15 days to keep them in prison until after the election. They say one of the latest was Anatol Lyabedzka, the leader of the United Civil Party, who was detained Wednesday on charges of disobeying police officers.

In Washington, the U.S. Commission on Security and Cooperation in Europe criticized the arrests as part of a pattern of intimidation ahead of the vote.

Meanwhile, authorities barred at least two Polish members of the official Organization for Security and Cooperation in Europe observer mission from entering Belarus after officials withdrew their visas.

Belarus President Alexander Lukashenko has ruled the former Soviet republic since 1994, and is seeking a third term.

The West has criticized him for his poor human rights record and for quashing political opposition. The United States has called Mr. Lukashenko Europe's last dictator.

Some information for this report was provided by AFP, AP and Reuters.