One witness said: "I heard a loud explosion and then there was black smoke. People started shouting but no one stopped the concert.
They just isolated the place around the explosion and emergency vehicles began coming," he said.
A criminal inquiry has been opened, with Minsk police chief Anatoly Kuleshov telling ITAR-TASS he suspected "hooliganism." Bomb experts were sifting through debris at the scene, Russian news agencies reported.
Some 2,000 musicians were due to perform in the massive "We are Belarussians" outdoor concert.