Tens of thousands of people are reportedly marching in the capital Minsk and other cities, in the last of several weeks of mass protests against President Alexander Lukashenko.
Large numbers of police were deployed, blocking key areas.
Police said they arrested around 250 people ahead of the protest, dubbed the March of Heroes.
The protests were sparked by a widely contested election a month ago and a brutal police crackdown that followed.
Protesters want Lukashenko to resign after alleging widespread rigging of the ballot.
But the Belarusian leader – in power for 26 years – has denied the allegations and accuses Western nations of intervening.
The 66-year-old has vowed to defend Belarus.
Most of the opposition leaders are currently under arrest or in exile.
This is the fifth Sunday in a row of mass protests, with around 100,000 rallies each week.
Eyewitnesses said the center of Minsk was inundated with people. They walk towards the elite residential neighborhood of Drozdy, where the country’s top officials, including President Lukashenko, live, but the roads are blocked by police.
Gatherings are also organized in Brest, Gomel, Mogilyev and other towns.
However, the Home Office said as of 3 p.m. local time (12 p.m. GMT), the protests did not affect more than 3,000 people across the country.
The ministry said arrests had been made in various parts of the capital and that those detained carried flags and placards “of an insulting nature”.
Video footage showed hooded men pulling people out of the crowds that were gathering for the start of the march and leading them to unmarked minibuses.
The protests were sparked by the August 9 elections, in which Lukashenko won a landslide victory amid widespread reports of vote rigging.
Violent clashes several nights after the polls led to thousands of arrests, and details have included severe beatings by police and overcrowding in detention centers.
This produced a new wave of protests, with weekend rallies drawing tens of thousands.
Lukashenko said he could forge closer ties with Russia, his main ally.
On at least two occasions in recent weeks, he has been photographed near his residence in Minsk, carrying a gun and surrounded by his heavily armed security personnel.