Large protests are expected to be held in Belarus later calling for long-time President Alexander Lukashenko to step down as a crackdown intensifies.
On Saturday, security agents in the capital Minsk detained dozens of people, mostly students, in the fourth weekend of protests.
In a separate march, thousands of women chanted “hands off the children”.
Meanwhile, a top opposition activist said she had taken refuge in Poland after threats from security forces.
Demonstrators say the presidential election of 9 August was rigged to keep Mr Lukashenko in office. Protesters, human rights activists and observers have accused riot police of brutally suppressing peaceful marches.
Mr Lukashenko, in power since 1994, has accused Western nations of interfering.