In a scene reminiscent of the movie The Italian Job, thieves have stolen some of Sweden's crown jewels from a cathedral and escaped in a speedboat.

They carried out the daring robbery in broad daylight, stealing priceless gold and jewel-encrusted crowns dating back to the 1600s after walking into a Swedish medieval cathedral, smashing open a glass security case before hopping onto bicycles and racing to a nearby lake, where they disappeared.

The two men vanished aboard a speedboat or jet skis after the noon heist into the vast patchwork of lakes around Strangnas, 60 kilometres west of the capital of Stockholm, police said.

The stolen artefacts included a gold crown and an orb dating to 1611 that were made for King Karl IX's funeral, as well as a jewel-encrusted crown dating to 1625 that was used in Queen Kristina's funeral. The stolen items were on display at an exhibition in the cathedral, and visitors were inside at the time.

"The alarm went off when the burglars smashed the security glass and stole the artefacts," Catharina Frojd, a spokeswoman for the 14th-century Strangnas cathedral, said.

The cathedral was displaying the crown jewels in an exhibition, and visitors were inside at the time of the heist. ( Pontus Stenberg )

The cathedral said on its website that the stolen pieces were kept "in accordance with the prevailing safety regulations in locked and alarmed displays in the cathedral". It gave no further details.

Police sent out a helicopter and boat to hunt for the thieves but found nothing. Authorities said no-one was hurt in the robbery but did not provide further details.

Tom Rowell, a visitor who was eating lunch outside, said he saw two men running from the cathedral toward a small nearby jetty where a motorboat was moored.

"The two men hurriedly jumped on board and it sped off," Mr Rowell said, adding that they "appeared non-Nordic". He did not elaborate.

Police spokesman Stefan Dangardt said "witnesses' testimonies varied quite a bit" and it was also possible the thieves escaped on jet skis.

The men used two stolen black bicycles equipped with baskets and a child's seat to race to the lake, Mr Dangardt added.

On Wednesday, divers were looking for clues in and along the shores of Lake Malaren, Sweden's third-largest freshwater lake. Police said the thieves could have fled further on jet skis.

A discarded bicycle near the scene of the robbery. ( Pontus Stenberg )

While the stolen artefacts are of great historic and cultural value, police expressed scepticism about whether the burglary would bring the perpetrators financial gain.

The stolen pieces are "impossible to sell" because of their uniqueness and high visibility, Maria Ellior of the Swedish police's National Operations Department told the Swedish news agency TT.

The theft would be logged at Interpol, enabling an international search, the agency said.

Strangnas, a small, quiet town with a population of about 13,000, is popular with Stockholm commuters and tourists, who come to see the cathedral and a street that has been called the prettiest in Sweden.

The Gothic-style cathedral, built between 1291 and 1340, is in the heart of the town. The cathedral's red-brick tower with a black top can be seen for miles around.

The cathedral was closed on Wednesday, and a grassy area by the jetty was cordoned off as police inspected the ground for clues.

The stolen artefacts are funeral regalia, which are placed inside or on top of a coffin to symbolize a deceased royal's identity and social ranking.

AP/ABC