The first explosion struck at 14:30 local time on December 19, 1943, killing those closest to the munitions immediately. German guards, ship crew, and Norwegian dockworkers died together on the wharf, though in death as in occupation, their fates would be recorded separately. Survivors scrambled for cover behind massive cement blocks lining the pier. Two hours of uncertainty followed, the damaged munitions smoldering and unstable. Then, at 16:30, roughly 400 tons of ammunition detonated in seconds. The blast hurled shells and grenades into the air, scattering them across the city. A nearby coal depot holding 20,000 tons ignited and burned for three weeks. Around 350 firefighters from Oslo and 60 from neighboring Aker fought flames that consumed entire blocks. The dangerous situation was not declared over until 21:45 that night.

The pressure wave radiated outward from Filipstad, shattering windows across a capital already dimmed by blackout curtains and occupation. The city architect tallied the damage: 53,000 square meters of shattered glass in residential houses, 28,000 in offices and shops, 9,000 in hospitals and schools. The total came to 90,000 square meters across 1,600 premises. Windows broke as far north as Skjennungstua in the forests of Nordmarka, kilometers from the blast site. Around 40 Norwegians died, along with approximately 75 Germans. Four hundred people were wounded. Some 400 buildings sustained severe damage. In December, with Norwegian winter setting in, thousands of homes suddenly lacked glass to keep out the cold.

The transport ship Selma survived the initial catastrophe. While fires consumed the wharf and ammunition cracked overhead, the vessel was towed away from the inferno at Filipstad. But escape proved temporary. On January 11, 1944, less than a month later, the Selma exploded and sank in a separate incident. Whether the original fire had compromised her cargo, or some other cause was responsible, the ship's delayed destruction added a grim coda to the December disaster. The German occupation authorities investigated and declared the Filipstad explosion an accident. Speculation about sabotage has persisted ever since, though no resistance organization or individual has ever claimed responsibility. The cause remains officially undetermined.

The explosion made an impression beyond the damage reports. Edvard Munch, Norway's most celebrated painter, living in nearby Ekely during the last year of his life, witnessed or was affected by the blast. His painting titled Explosion captures the event in swirling, violent color. Munch died just weeks later, on January 23, 1944. Today the Filipstad district has been redeveloped into a modern waterfront area, its wartime past largely invisible beneath new construction. The explosion left no permanent memorial at the site, though it remains one of the largest accidental detonations in Scandinavian history. For occupied Oslo, the blast was one more catastrophe endured under foreign control, where even the investigation into why it happened belonged to the occupiers rather than the occupied.