'Fearing the worst'
Oistein Mjarum, head of communications at the Norwegian Red Cross, told BBC World TV the blast could be heard across the city.
"There are a lot of people injured but we don't know how many," he said.
"This is a very busy area on Friday afternoon and there was a lot of people in the streets, and many people working in these buildings that are now burning, so we fear the worst but we have no confirmed details of casualties or the number of dead."
Amid reports of a bomb attack, finansakrobat likened the explosion to the 9/11 attacks on America.
Christian Aglen, development manager of Norwegian financial news website E24.no, tweeted that it was "undoubtedly one of the worst days in Norwegian history".
This news could get worse