Thieves grabbed jewels and other treasures worth up to €1 billion from an eastern German museum in the early hours of Monday, Bild newspaper reported.
The intruders cut the electricity supply in Dresden's Gruenes Gewoelbe, or Green Vault museum, which houses one of Europe's largest collections of jewelery and court riches, the newspaper said without giving a source.
Police sealed off the building in Dresden's Baroque city palace and said they were still trying to work out what was missing. We have not identified a perpetrator and nor have we yet made any arrests, police spokesman Marko Laske said.
There was no immediate comment from museum staff.
The collection was founded in the 18th century by August the Strong, Elector of Saxony and later King of Poland.
One of its best known treasures - the 41-carat Dresden Green Diamond - was away on loan to New York's Metropolitan Museum of Art at the time of the break-in.
Other exhibits in Dresden include a table-sized sculpture of an Indian royal court from the 18th century, made out of gold, silver, enamel, precious stones and pearls.
Another is a 1701 golden coffee service by court jeweler Johann Melchior Dinglinger, decorated with lounging cherubs.
The treasures of the Green Vault survived Allied bombing raids in World War II, only to be carted off as war booty by the Soviet Union. They were returned to Dresden, the historic capital of the state of Saxony, in 1958.
The theft was a blow to the whole state, its premier, Michael Kretschmer, said.
The works in the Green Vault and the Palace were built up by the people of Saxony with many centuries of hard work, he said.