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.
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”.
Video footage showed
gathering for the start of the march and taking them to unmarked minibuses.
Protests were triggered by elections on 9 August, in which Mr Lukashenko was handed an overwhelming victory amid widespread reports of vote-rigging.
Violent clashes on several nights following the poll led to thousands of arrests, and details emerged of severe beatings by police and overcrowding in detention centres.
This produced a new wave of demonstrations, with weekend rallies drawing tens of thousands.
Mr Lukashenko has said he may establish closer ties with Russia, his main ally.
On at least two occasions in the past few weeks, he has been photographed near his residence in Minsk carrying a gun and being surrounded by his heavily armed security personnel.