They accuse the authorities of widespread vote rigging in the presidential election on Sunday.
The BBC reports police have been trying to disperse the demonstrators.
Eaerlier, Vladimir Neklyayev, an opposition presidential candidate, was beaten unconscious shortly before the poles closed.
Exit poles suggest that President Alexander Lukashenko will win a fourth term having secured more than 70% of the vote.
He has been in power since 1994.