The rally, which was mostly attended by people in their retirement age, began in the Belarusian capital on Monday afternoon. Initially, the protesters gathered at the Independence Square near the House of Government, from where they marched along Independence Avenue.
"There were stun grenades and tear gas used when citizens began to show aggression against law enforcement officers", Lashkevich said.
The number of those detained at an unauthorised mass event is being identified, the spokesman added.
Earlier, the country's Interior Ministry said that the protests in Belarus have redicalised and the law enforcement will use special equipment and live rounds if necessary.
Mass opposition protests have persisted in Belarus following the disputed 9 August presidential election, in which longtime President Alexander Lukashenko claimed a sixth term. Hundreds of people were injured during the riots, including over 130 law enforcement officers. Three protesters died.