No one was injured when what appears to have been a small explosive device was thrown at the ground floor room of a residence in Lija on Wednesday evening.
The incident happened at about 7.20pm in a square in St Andrew Street. Somebody hurled the device through a window pane and fled, neighbours said.
The device exploded but no one was injured.
The incident happened metres away from the residence of the German ambassador.
The police said minimal damage was caused and they were investigating what had happened.
The area was closed off by policemen from the Rapid Intervention Unit and investigations were made on site by army bomb disposal experts. Neighbours were evacuated while the initial investigations were made.