Anatol Lyabedzka, who heads the United Civic Party, said he was seized by "seven or eight" plain-clothes policemen.
The opposition demonstration he was heading for was to mark the seventh anniversary of the disappearance of opposition politician Yury Zakharanka.
Lyabedzka was released after three hours. Lyabedska says he believes the police detained him in the hope that his absence would prompt the opposition to cancel the protest.
Twelve other people were also detained before the demonstration. All were subsequently released.
Minsk, March 25
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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