Burglars have stolen three diamond jewellery sets from one of Europe's largest treasure collections - the Dresden Green Vault in eastern Germany.
The historic sets consist of 37 parts each, and there are fears the thieves may try to break them up.
Officials are still trying to establish exactly how much was stolen in the break-in early on Monday.
Saxony's ruler Augustus the Strong created the collection in 1723 in what is one of the world's oldest museums.
"Three out of 10 diamond sets have gone," said Marion Ackermann, head of the Dresden state museums.
The stolen sets from the Green Vault (Grünes Gewölbe) are reported to also include some rubies, emeralds and sapphires.
The thieves - still on the run - removed part of an iron grille on a ground-floor window, then smashed the glass.
About 5am local time on Monday, firefighters were called to tackle a blaze in a nearby electricity junction box.
There is speculation the fire disabled the museum's alarm system. It put out some of the street lights.
Police are examining CCTV footage that shows two suspects in the dark, but more people may have been involved in the robbery.
A car found burning in Dresden early on Monday local time may have been the getaway vehicle used by the burglars, police said.
The museum had guards on duty at night, Ms Ackermann said.
Dozens of police cars were at the scene and the Green Vault in central Dresden was now shut.
"Three out of 10 diamond sets have gone," said Ms Ackermann.
However, Dresden police and museum officials did not provide any further details. Ms Ackermann described them as "priceless - we can't put a figure on it".
Cannot be sold
"The items cannot be sold on the art market legally - they're too well known," she said.
Ms Ackermann stressed that the cultural value of the unique collection was far greater than its material value.
The popular German daily Bild said the thieves had grabbed jewels worth €1bn ($1.7 billion).
The collection is housed in eight ornate rooms in the Residenzschloss - a former royal palace.
Three rooms were destroyed by Allied bombing in World War II but the museum was restored to its previous glory after the war.
It is called the Green Vault because some rooms were decorated with malachite-green paint.
The most valuable items are in the palace's historic section on the ground floor.
There are about 3000 items of jewellery and other treasures decorated with gold, silver, ivory and pearl. They include a figure of a moor studded with emeralds and a 648-carat sapphire - a royal gift from Russia's Tsar Peter the Great.
One of the most valuable jewels is a 41-carat green diamond currently on show in New York.
The collection was founded by Augustus the Strong. He was Elector of Saxony (a German prince entitled to take part in the election of the emperor) and later king of Poland.
Saxony's minister-president, Michael Kretschmer, voiced outrage at the crime and said: "Not only were the state art collections burgled, but the people of Saxony too.
"The valuables housed in the Green Vault and Residenzschloss were acquired by people in the Saxony Free State with difficulty, over many centuries." He said the collection was an integral part of Saxony's history.