A Reuters witness saw a column of people numbering at least 10,000 moving towards Independence Square on Sunday, where the government and Belarus President Alexander Lukashenko sit.
On Saturday, at least 5,000 people had marched through the city demanding the release of a jailed opposition leader, in the latest in a wave of mass protests after the Aug 9 presidential vote.
A total of 114 people were held across the country last night, of which 99 were detained in the capital, the Interior Ministry said.
Oktyabrskaya Square in central Minsk was fenced off with barbed wire on Sunday, with armed law enforcement forces seen behind it, a Reuters witness said. Independence Square was also fenced off.
Citing the Minsk police, Russian state newsagency TASS said on Sunday that detentions had started in the capital ahead of the protests due from 1100 GMT.
One protester who gave her name as Tatiana told Reuters, "there were over 1,000 people in our group."
"Soldiers rounded us up in several circles, people were selectively pulled out of the crowd and beaten," she added.
Olga Chemodanova, an official at the Interior Ministry, told TASS separately that additional police and interior ministry forces were called to the streets of Minsk on Sunday to prevent the holding of what she described as an unauthorized rally.
Belarus, Russia's closest ex-Soviet ally, has seen mass protests since the Aug 9 presidential elections, which the opposition says were rigged.
Lukashenko, who has been in power for 26 years, denies this and has said foreign powers are behind the protests.Belarus' key opposition figures have been either jailed or forced out of the country. Lukashenko is due to visit Russia for talks with President Vladimir Putin on Monday and both countries start joint military drills on Sept 14.