Fashion Zinc Alloy Earringused gym equipmentOistein Mjarum, head of communications for the Norwegian Red Cross, which has offices nearby, said the blast could be heard across Oslo.
"This is a very busy area on Friday afternoon and there were a lot of people in the streets, and many people working in these buildings that are now burning," he said.
Local resident Silvio told the BBC the blast shook everything in his apartment and that he saw several unconscious people in the street.
"If they were dead or not I wouldn't be able to tell you but they were receiving assistance at the time," he said.
Mr Mjarum said people across Oslo and Norway were in shock.
"We have never had a terrorist attack like this in Norway - if that's what it is - but of course this has been a great fear for all Norwegians when they have seen what has been happening around the world."
The United States has condemned the "despicable acts of violence" in Oslo, while the President of the European Council, Herman Van Rompuy, said he was "deeply shocked" by "these acts of cowardice for which there is no justification".