A human rights group in Belarus said more than 500 people were arrested in protests around the country calling for authoritarian President Alexander Lukashenko to step down.
The Sunday demonstrations continued the wave of near-daily protests that have gripped Belarus since early August.
In the capital Minsk, police wielded clubs and used tear gas and water cannon to disperse thousands of demonstrators.
The Viasna human rights organisation reported detentions at demonstrations in other cities, including Vitebsk and Gomel. It said the nationwide arrest numbers exceeded 500.
Many of the demonstrators carried placards in commemoration of Raman Bandarenka, an opposition supporter who died on Thursday after reportedly being beaten in police detention.
A wide wave of protests, some attracting more than 100,000 people, broke out following the August 9 presidential election that official results say gave Mr Lukashenko a sixth term in office.
The opposition and some poll workers say the results were manipulated.
Mr Lukashenko, who has suppressed opposition and independent media during 26 years in power, refuses to negotiate with the opposition and alleges the protests are incited by Western countries.