Three armed robbers stormed into Stockholm's waterfront National Museum on Friday, made off with a Rembrandt self-portrait and two masterpieces by Renoir, then fled in a waiting boat. One of the art thieves stood in the lobby with a machine gun while the other two men grabbed the paintings in two different rooms, a police spokesman said. Fleeing, the robbers threw nails in front of the museum, police said. The boat the men used was found at Sodermalm, a neighborhood just south of Stockholm. The Renoirs were titled ''Young Parisian'' and ''Conversation with the Gardener,'' police said. The title of the Rembrandt was not immediately clear; its dates from 1630. The paintings are worth millions of dollars, acting museum director Torsten Gunnarsson told The Associated Press. He did not give an exact figure. Scores of policemen had been assigned to the case. The National Museum's collections contain about 15,000 paintings and sculptures. One of the most valuable paintings is Rembrandt's ''Kitchenmaid.'' Thomas Hall, an arts professor at Stockholm University, said there is no open market for the paintings and ''the whole robbery is absurd.'' ''The paintings will eventually return to the museum,'' he said. '' No collector wants to buy something that you can't show.'' The biggest art theft in Sweden happened at Stockholm's Modern Museum in November 1993 when works by Picasso and Georges Braque worth about $48 million were stolen, AP reports.