According to police reports, two masked robbers entered the museum around 11am on Sunday while tourists and other guests strolled around in the building in Tøyen in Oslo.
"Two men dressed in dark clothes threatened the guards with handguns," Kjell Pedersen of the Oslo police told TV 2 Nettavisen. "They stole Madonna and The Scream. They took them off the wall and walked off."
The Scream is one of four versions painted by Edward Munch in 1893. The ghostly, agonised figure against the background of a red sunset is one of the most well known images in the world of art - a symbol of despair and alienation. The estimated value for the two famous paintings are NOK 650 million (USD 96.8 million).
Nationwide hunt
Large police forces were Sunday afternoon looking for the robbers and have asked the public for help.
"We have technical leads from the museum and in the car which we are working on right now," said Kjell Pedersen at the Oslo police district. "At the same time, we are conducting tactical investigation all over the city."
Pedersen did not want to speculate whether or not any particular group are behind the robbery, or if the robbers are from Norwegian or international group, but he stated that one of the robbers spoke Norwegian.
Director Sune Nordgren at the National Art Museum believes that an international league was behind the armed robbery at Oslo's Munch Museum on Sunday. He now expects ransom to be demanded for the safe return of national treasures The Scream and Madonna.