Two people have life-threatening injuries after the incident, which was a "deliberate attack," according to Prime Minister Christophe Mirmand.
"This is, as far as I know, the first time in history that something like this has happened in the principality (of Monaco)," he says.
The explosive charge reportedly contained both bolts and shotgun pellets. Another person, a child in his early teens, has minor injuries.
According to prosecutors, a suspect left a bag or package in the lobby of the building. At this time, no suspect has been taken into custody.
