Detail of Jeune Parisienne by Renoir

On Saturday, police received photographs of the stolen paintings and a demand for "several million kronor" for their return.

Police would not say exactly how much the ransomers requested. A million Swedish kronor is equivalent to about $100,000.

Request for aid

Sweden has asked Interpol to join the search for the suspects, and police forensic scientists have been combing the museum for clues.

The three thieves walked into the museum in central Stockholm as it was preparing to close for the day.

Rembrandt self portrait from 1630

One, armed with a machine gun, held up security staff while the other two ran to different rooms upstairs

After ordering guards there at gunpoint onto the ground, the two thieves went straight for their target paintings. Within moments all three had left the building and sped off into the darkness on a small motor boat waiting for them near the museum.

Possible accomplices

Other people are also thought to have been involved in the crime, as two phone calls reported car fires in the area at the same time that the gallery alarm went off at police headquarters.

This was professional work

Police Inspector Thomas Johansson

The subsequent confusion apparently gave the robbers the time they needed to escape.

Police combed the capital in boats, cars and on foot.

About two hours after the theft, investigators recovered the getaway boat on the banks of the Malar lake in southern Stockholm.

It was taken to a laboratory to be checked for fingerprints and other clues.

Detail of Conversation with the Gardener by Renoir

The works stolen were: Self Portrait (1630) by Rembrandt, and two paintings by Renoir - Conversation with the Gardener and Jeune Parisienne. They were each valued at between $7.5m and $10m (75 million and 100 million Swedish kronor).

None of the paintings were insured.

The theft is the biggest in Sweden since robbers cut through the roof of Stockholm's Modern Museum in 1993 and stole eight works of art by cubist masters Pablo Picasso and Georges Braque worth about $60m. Most of the works were later recovered.