Thieves have stolen two crowns and an orb from the Swedish royal family's collection, making off in a motorboat after the heist.
The priceless artefacts were snatched from a cathedral in Strangnas, about an hour west of the capital Stockholm, at lunchtime on Tuesday, police said.
The robbers broke into a glass case that held the funeral regalia of King Karl IX and his wife Queen Kristina and swiped the two crowns and an orb from the 17th century.
They then made their getaway in a motorboat that was waiting just a couple of hundred meters away on lake Malaren, Sweden's third biggest lake, police said.
No arrests have yet been made and no suspects had been identified but police are now publicising the theft to try to recover the valuables.
"We want to spread information and pictures of these items so that they can be identified as stolen objects," police spokesman Thomas Agnevik said.
The thieves and the jewels are being sought internationally via Interpol, police spokesman Stefan Dangardt said, noting the objects were a "national treasure" and would likely be "very difficult to sell".
"Several people were seen leaving the church by boat or waterscooter after the theft. We have spoken to witnesses, but we are interested in further information from anyone that has made any observations," he said.
King Karl IX died in 1611 and his wife Queen Kristina in 1625. The items are priceless and police said that the objects would be very hard to sell on the open market.
"What usually happens with this type of object is that they are recovered sooner or later, because there are very few people who are prepared to handle such items," Agnevik said.
"We have high hopes of getting them back."
"There is of course going to be a lot of media interest in these types of objects. There will be pictures in the media. It's just not possible to sell these kinds of objects," the national police coordinator for thefts of cultural artifacts, Maria Ellior, told news agency TT.
"So we can only speculate about (the thieves) intentions," she added.