RFE/RL's Belarus Service reported that opposition leader Alyaksandr Kazulin, who was leading the march, was detained.
As police charged the crowd, there were a series of explosions of undetermined origin. A woman who was among the protesters described to RFE/RL's Belarus Service how the police assault started.
"We were attacked by riot police operating under the command of a colonel," she said. "They started beating people with truncheons, six against one. They're beating children."
The clashes erupted after thousands of opposition supporters had peacefully rallied on Yanka Kupala Square to demand that the outcome of the March 19 presidential election be annulled.
THE AUTHORITIES GET TOUGH: RFE/RL's Belarus Service filed these images from the police action against the March 25 demonstration in Minsk. Photographs by Maks Kapran.
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