Tens of thousands of people are reportedly 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 are marching towards the elite residential area of Drozdy, where the country’s top officials including President Lukashenko live, but roads are blocked by police.
Rallies are also being held in Brest, Gomel, Mogilyev and other cities.SUV exploded in the center of Belgrade, there are casualties