Norway police have said the explosion was caused by a bomb. The tangled wreckage of a car was visible outside one building.
No-one has claimed responsibility.
euronews spoke to Roger Bruland, head of the international desk at the Norwegian public broadcaster NRK:
“This doesn’t look like Oslo any more it looks like some foreign country where you see this massive destruction. The government building you see at least a part of it, all the windows are broken and there is a lot of debris on the road. We have military police there, we have police, ambulances and it looks like a war zone, really.
“This is huge for Oslo. Oslo is peaceful as you say but this is something really massive and destruction on massive scale.
“There are no people there now they have all been evacuated and there have been scenes of chaos and some panic in the streets of Oslo today.
“People actually here at NRK heard the explosion and we are talking about four kilometres distance and people that are living in the downtown area alsio heard the explosion so the explosion was big.”