The Belarus government has ordered eight members of a Scandinavian team of unofficial election observers to leave.
The two Swedes and six Danes were part of a monitoring team sent by the unaccredited Danish group Silba.
They were arrested on Tuesday - the first day of early voting ahead of a presidential election officially set for Sunday.
Opposition groups say early voting is an opportunity for the president, Alexander Lukashenko, to rig the poll.
The Swedes, Thomas Ochman and Bjorn Stenstrom - members of Sweden's Liberal Party - were arrested in the western city of Grodno after visiting a polling station there.
They are accused of breaking a law which states that conducting opinion polls is banned in Belarus.
Mr Ochman said six other Silba representatives were arrested in Minsk.
The team is not part of the election monitoring mission of the Organisation for Security and Cooperation in Europe.