Belarussian riot police have swooped on around 300 protesters against the re-election President Alexander Lukashenko's camped out in Minsk's October Square.
The demonstrators were taken away in trucks to a detention centre pending trial. It was the early hours of the morning at the time of writing.
The demonstrators were pressing for a re-run of the March 19th poll which gave Mr Lukashenko five more years in power in the ex-Soviet state.
The opposition have the support of the international community - bar Russia - in claiming the poll was blatantly rigged.
The United States and the European Union issued separate statements saying they planned to impose restrictions on Belarus, including a travel ban, in the wake of the election.
Despite the overnight arrests, opposition leader Alexander Milinkevich said he would not be deterred from holding a peaceful rally - also unauthorised - in the morning as planned.
If authorities sealed off October Square, protesters would move to a different location which he refused to disclose, he said.
As night fell, Belarussian police prevented dozens of residents from approaching the square where activists had been gathering. Police at the site were considerably more aggressive in their approach than over the past four days.
It was not immediately clear what support Mr Milinkevich could expect for Saturday's demonstration, which will also mark the independence day of a short-lived Belarussian republic in 1918.
Dissent is normally quashed quickly in the tightly-policed ex-Soviet state. But authorities have handled these protests with comparative tolerance.
Mr Lukashenko won the election with an official tally of 83 percent to 6 percent for Mr Milinkevich. The Organisation for Security and Cooperation in Europe (OSCE) monitored the election and declared far below acceptable standards of fairness.