Tens of thousands of people have 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 arrested about 400 people ahead of and during the protests, 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”.
No sign of enthusiasm dwindling
Analysis by BBC correspondent Jonah Fisher
In many ways Sunday’s demonstration was similar to previous weeks.
When the march was at full strength the riot police had little choice but to watch on as the protesters filled the streets and waved their red and white flags.
The now famous “Goose for a free Belarus” was there, a bow tie round its neck, flapping its wings and posing for selfies. Plenty of families came too, determined to enjoy the warm weather.
It’s on the side streets, and in the exposed moments when people arrive and disperse in smaller groups that the security forces strike.
It’s not dignified or disciplined. The police, their faces usually covered, launch crude tackles at the protesters before dragging them kicking and screaming into waiting minivans.
After five Sundays of huge demonstrations there’s still no sign of enthusiasm dwindling or that the threat of violence is stopping people from coming.
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.