The cause of the blast was unknown but the tangled wreckage of a car was outside one building. Police and fire officials declined to comment on the cause. Injured people were seen lying on the street in pools of blood.
The 17-storey building, which also houses Norway's biggest tabloid newspaper VG, was reported to be on fire, with thick black smoke being seen for miles.
Witnesses reported the massive blast blew out most windows in the building as well as nearby government departments including the oil ministry, in Norway’s capital and most populated city.
The blast, which occurred just after lunch on Friday, left at least a dozen people. “Some people covered with blood are lying in the street.” Other offices are being evacuated.