A Reuters cameraman at the scene said crack police units surrounded the demonstrators in a central square in the capital, Minsk, loaded them on to trucks and took them away.
The demonstrators had rejected police calls to leave October Square.
One of the detained protesters called Reuters by mobile phone and said they were being taken to a pre-trial detention centre in the capital.
The unprecedented protest in the former Soviet state began on Sunday after Alexander Lukashenko was declared the landslide winner of elections branded as flawed by international monitors.
Despite his 12 years of Soviet-style rule that has made him a pariah in the West, Lukashenko is genuinely popular among the 10 million Belarussians for having ensured relative political and economic stability.
A son of Alexander Milinkevich, the opposition leader, was among about 400 protesters cleared from Minsk's central square by the riot police in the early hours of Friday morning, the opposition leader's wife said.
"Our son is among them and we're going to try to go to where they're being detained," Inna Kuley said.