More than 20,000 treasures from the House of Hanover are being auctioned by descendants of the German royal family.
Kings' goblets, Chinese tapestries, Russian porcelain and paintings by Italian masters are on offer in a sale expected to fetch 13m euros (£9m).
The auction is being held to raise funds for the restoration of the fairytale Marienburg Castle, in northern Germany.
The castle is the ancestral home of Prince Ernst August.
Prince Ernst, 51, is the grandson of the last German Emperor, William II, who was the grandson of Britain's Queen Victoria.
UP FOR SALE
He is married to Princess Caroline of Monaco.
"It is the last auction of a royal family in Germany, perhaps even on the continent," said Christoph Douglas, cultural advisor to Prince Ernst.
But the prince's younger brother, Heinrich, said he was "speechless" about the auction.
"It is a total sell-out," he told SWR television.
"My brother has not understood what tradition means. He wants to do something radical and live the good life abroad."
Painting sold
But thousands of bidders have been tempted by the prospect of getting their hands on the royal heirlooms and have flocked to Marienburg.
Some artefacts will have more historical significance than others.
Some 18th Century German silver kettle drums, for example, are adorned with the coat of arms of George III, who was king of Britain and Ireland as well as Hanover.
A triptych painting from a student of Lucas Cranach has already fetched 180,000 euros (£122,000).
The auction runs until 15 October.
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