Thieves have broken in to the fabled Fontainebleau castle south of Paris and stolen 15 or so "priceless" Chinese artefacts in a lightning raid, French authorities said.
The theft took place in "one of the most secure parts of the castle, equipped with alarms and surveillance cameras," said the French culture ministry.
Among the items taken was the crown of the King of Siam, presented to Emperor Napoleon III during a visit from the Siamese ambassador, the ministry said.
Jean-Francois Hebert, who runs the Fontainebleau castle, said it was a "very big shock and trauma."
"We think they were very determined, knew exactly what they were looking for and worked in a very professional manner," added Hebert.
The objects taken were "unique" and "priceless", he added.
"They were among the most beautiful pieces in the museum," said Hebert.