THIEVES in Sweden walked into a small town’s medieval cathedral in broad daylight and made off with priceless crown jewels dating back to the early 1600s before escaping by speedboat.

Two men vanished into lakes around Strangnas, 37 miles west of Stockholm, at around noon on Tuesday.

The thieves snatched two gold crowns and an orb made for King Karl IX and Queen Kristina in the daring robbery.

One of the crowns is encrusted with precious stones.

The stolen items were on display at an exhibition in Strangnas 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 cathedral, said.

The church wrote on its website that the stolen items were kept "in accordance with the prevailing safety regulations in locked and alarmed displays in the cathedral".

Police sent out a helicopter and boat to hunt for the thieves but found nothing.

Authorities said nobody was hurt in the robbery.

Tom Rowell, a visitor who was eating lunch outside, said he saw two men running from the cathedral towards 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".

Mr Rowell is getting married at the cathedral next weekend.

"It's despicable that people would steal from a holy building," he said.

While the items are of great historic and cultural value, police expressed doubt as to whether the burglary would bring the perpetrators financial gain.

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

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

Strangnas is a small, quiet town with a population of about 13,000.

It 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 from 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.

Police also questioned witnesses who were inside the church at the time of the theft and people outside who saw the suspects get away.