The UN's human rights envoy for Belarus has urged authorities to free all political prisoners, condemning alleged rights violations after Sunday's poll.
Protesters continue to brave freezing temperatures in Minsk
Adrian Severin said more than 100 people had been detained following protests against President Alexander Lukashenko's election victory.
Meanwhile several hundred protested for a fifth day in the capital, Minsk.
The protests came as official election results were declared, giving Mr Lukashenko 83% of the vote.
The main opposition candidate, Alexander Milinkevich, received just 6.1%, the figures showed.
The protesters accuse Mr Lukashenko of rigging the presidential poll. They are demanding a new vote.
International monitors have condemned the poll as "severely flawed" and the EU has said it may impose sanctions. But Russia says the poll was fair.
Mr Lukashenko has said he believes voters have made their choice and has vowed to crush any attempts to launch a revolution.
In a statement issued in Geneva, Mr Severin said he was concerned about the "detention of numerous citizens peacefully demonstrating against the electoral results".
Mr Lukashenko insists the elections were fair
The initial demonstration on Sunday evening drew around 10,000 people to the square.
More than 100 people were arrested and many others were turned away from the square by police, reports say.
The protests later dwindled to just a few hundred, but opposition leaders have called for another big demonstration on Saturday to coincide with the anniversary of the short-lived Belarussian republic of 1918.
Mr Severin called on Belarus to "release immediately and unconditionally all political prisoners, and to bring all violations of freedom of expression and of the right of peaceful assembly to an immediate end".
The call coincided with the release from prison of another opposition leader, Vintsuk Vyachorka, who had served 14 days for organising an unauthorised rally.
Mr Vyachorka said there were 180-200 prisoners which he described as political at the prison.