Tens of thousands of people have reportedly been marching in the capital Minsk and other cities, in the latest of several weeks of mass protest against President Alexander Lukashenko.
Large numbers of police have been deployed, blocking key areas.
Police said they had arrested about 250 people ahead of the demonstration, dubbed the March of Heroes.
The protests have been triggered by a widely disputed election a month ago and subsequent brutal police crackdown.
Demonstrators want Mr Lukashenko to resign after alleging widespread ballot-rigging.
But the Belarusian leader – in power for 26 years – has denied the allegations and accuses Western nations of interfering.
The 66-year-old has promised to defend Belarus.
Most opposition leaders are now under arrest or in exile.
It is the fifth successive Sunday of mass protests, with about 100,000 rallying each week.
Eyewitnesses said the centre of Minsk was flooded with people. They marched on the elite residential area of Drozdy, where the country's top officials including President Lukashenko live, but were blocked by police.
Rallies are also being held in Brest, Gomel, Mogilyev and other cities.
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However, the Interior Ministry said that as of 15:00 local time (12:00 GMT) the protests involved no more than 3,000 people across the country.
The ministry said arrests were made in various districts of the capital, and that those detained were carrying flags and placards "of an insulting nature".
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