The blast took place at the Airport Road Customs Intelligence office in Quetta.
One person was injured in the blast and was rushed to a nearby hospital for medical treatment. The building caught fire after the blast.
Police said that the nature of the blast was being ascertained, security forces had also reached the spot after the blast and a bomb disposal squad had also been called.
A security guard at the customs office told police that three armed men entered the office and took the staff hostage. The gunmen tied the hands of the staff and threatened serious consequences. Done.