Opposition leader Alyaksandr Milinkevich said he will defy the warning from prosecutors and he will lead the rally on October Square, the site of protests last month against the reelection of President Alyaksandr Lukashenka.
Authorities in Minsk gave permission for a rally but warned demonstrators to stay away from October Square, which is in the center of the city.
The opposition has traditionally gathered on the anniversary of the Chornobyl disaster, the world's worst nuclear accident, to protest against Lukashenka's handling of the consequences of the catastrophe.
Busloads of police entered streets near the square hours before the rally was due to start, and police vans line parts of central Minsk.