London-based Amnesty International, citing figures from the Belarusian rights group, Vyasna, said in a written statement that at least 128 people are believed to have been detained since the elections on March 19.
The group estimated that the number of arrests could be as high as 400 nationwide.
The statement said large numbers of peaceful demonstrators have been detained and reportedly beaten by security forces in an apparent attempt to intimidate demonstrators.
AI called for the immediate release of all people detained for the legitimate and peaceful expression of their views.
In Minsk, thousands of opposition supporters have been spending a fourth consecutive night in central Kastrychnitskaya (October) Square to protest the reelection of President Alyaksandr Lukashenka to a third term.
The opposition, backed by much of the international community, alleges that the election was neither free nor fair and that a new vote should be held.