The demonstrators, draped in red-and-white opposition flags, descended on Independence Square, chanting slogans against President Alexander Lukashenko, who secured victory in a controversial election earlier this month.
Local news outlets published videos on social media showing water cannon and riot police with shields moving towards Independence Square.
So far, authorities have arrested nearly 7,000 people, with protest organizers alleging police of torturing and abusing the detainees.
The Defense Ministry announced it will deploy the army to protect national monuments from protesters and any unrest near these sites will not be tolerated.
The ministry said it would intervene to protect World War II memorials, describing them as "sacred places," and ordered the closure of four metro stations in central Minsk.
"The news [of the army deployment] has outraged people here. So far only the riot police have been used against the protesters. It is unclear whether these soldiers will be willing to use force against unarmed demonstrators," said the journalist.
On Saturday, Belarus' BelTA state news agency cited Lukashenko as ordering his defense minister to enact "the most stringent" army measures while visiting military units at Grodno, near the country's border with Poland.
The autocratic six-term president again claimed NATO troops in Poland and Lithuania were "seriously stirring" near Belarus's borders — assertions denied by trans-Atlantic leaders in recent days.
Belarus' army must "protect the territorial integrity of our country," said Lukashenko, adding "military support is evident," a reference to NATO.