The blast took place before dawn at the concert in downtown Minsk marking the ex-Soviet nation's independence day.
Minsk police spokesman Alexander Lastovsky said authorities had opened a criminal probe on charges of "hooliganism." "It was an action by hooligans," he said.
Lastovsky said over 20 people were hospitalized, but the Health Ministry put that number at more than 50.
Belarusian Emergency Situations Ministry spokesman Alexei Usatov said the bomb went off at around 12:30 a.m. The concert was held at the Hero City memorial, which commemorates Minsk's suffering during World War II.
The explosion was an unprecedented event in Belarus, which has been tightly controlled by authoritarian President Alexander Lukashenko, who has been described in the West as "Europe's last dictator."