President Alexander Lukashenko, who has cracked down on opposition and the news media in his 21 years of rule, is certain to win a new term in Sunday's vote.
Police did not interfere with the march and protest rally in Minsk on Saturday.
Rally participants carried placards reading "Stop Lukashenko" and "They have stolen the election from us." Officials say about 30 percent of the electorate voted in early balloting, which opponents say is rife with potential fraud.
Demonstrators also carried portraits of Svetlana Alexievich, the Belarusian author and Lukashenko critic who won the Nobel literature prize on Thursday.