A powerful bomb rocked Norway's government headquarters in the capital of Oslo on Friday, killing at least two people and injuring 15 others.

The powerful blast also set the nearby oil ministry building on fire. Hundreds of windows in the 17-story government headquarters were shattered, as were others in buildings as far as 400 meters away. Thick, black smoke billowed from some of the offices, and streets in the normally quiet neighborhood were littered with debris from the explosion.

Hours later, Oslo police reported that a gunman disguised as a police officer opened fire on youths attending a camp sponsored by the ruling Labor Party. At least five people were wounded in the attack. Authorities said they dispatched anti-terror police to the campsite on Utoeya island south of Oslo. It is not known whether there is a link between the bomb blast and the shootings.

The headquarters building in downtown Oslo houses the office of Prime Minister Jens Stoltenberg, but a Norwegian spokesman said he and his staff were not injured. Police said two people were killed in the mid-afternoon blast.

Mr. Stoltenberg told a Norwegian television station that the explosion was a “serious situation.” The United States called the explosion a “despicable” act of violence, while European Union President Herman Van Rompuy condemned it as an act of “cowardice.”

Norwegian State Secretary Hans Kristian Amundsen told the BBC that there are people trapped inside the headquarters building but he declined to elaborate.

Oslo police said the explosion was caused by a bomb. The tangled wreckage of a car could be seen outside one building, possibly indicating a car bomb was detonated.

Witnesses said they saw several injured people covered with blood as they were carried or ran from government headquarters. One witness said “people ran in panic.”

Political violence is virtually unknown in Norway, and in Oslo, where the Nobel Peace Prize is awarded annually. But it is not immune from the types of terrorism cases linked to Islamic terrorism that are common in Western nations.

A Norwegian prosecutor last week filed charges against an Iraqi-born cleric, accusing him of threatening to kill a Norwegian politician if he is deported from the Scandinavian country. But State Secretary Amundsen cautioned that officials are “not speculating anything” about who might be responsible for the attack.