At the moment, the explosion appears to have been caused by a car bomb, but that has not been confirmed by officials.
UPDATE: Reuters is reporting at least one person has died.
UPDATE 2: A police officer on-scene has told Bloomberg that a bomb caused the blast.
UPDATE 3: Reuters confirms two deaths in the blast.
UPDATE 4: Contact information: Relatives can call 81502800. Witnesses can call +47 22669966.
UPDATE 5: There are reports, which have been confirmed by police, of a man dressed as a police officer opening fire at Utøya in Buskerud. Police are now on-site and have sealed off the area, but there are people hiding from the gunman in the sealed zone, and it is not known how many have been injured, if any.
UPDATE 6: The Guardian's live blog of events is here.
UPDATE 7: It is now being reported that five have been injured in the shooting. BBC coverage is here.
UPDATE 8: Oslo authorities are urging people to stay at home.
UPDATE 9: Per Reuters, seven people have died. Two more severely injured.
UPDATE 10: Reuters:
A gunman detained after shootings at a youth meeting on a Norwegian island and a bombing in central Oslo on Friday is Norwegian, Justice Minister Knut Storberget said.
"Our information is that he is Norwegian," he told a news conference. "I don't know so much about him."
He said that seven people were killed in a bombing in central Oslo and 10 injured. Several people were killed and others wounded in a shooting at a youth camp northwest of Oslo of the ruling Labour Party, he added.