The display case from which the sketchbook was stolen
A sketchbook of some 32 drawings by Pablo Picasso worth about 8m euros ($11m; £6.8m) has been stolen from a museum in Paris, police have said.
The theft from the Picasso Museum was discovered on Tuesday afternoon but the exact time and circumstances have yet to be determined.
Initial investigations showed the sketchbook was held in an unlocked display case on the first floor.
Picasso is regarded by many as the greatest artist of the 20th Century.
Police sources told the Agence France-Presse news agency that the theft would probably have been committed between Monday evening and noon on Tuesday.
Picasso is regarded by many as the greatest artist of the 20th Century
There were no signs of a break-in an no alarms were set off.
The museum was closed at the time although there was a private viewing on Tuesday.
The museum's management have so far refused to comment on the theft.
A special unit of the interior ministry, the BRB, has taken over handling of the case.
An art insurance expert said the theft appeared to be "ill conceived and opportunistic".
"It will be nearly impossible to shift the sketchbook on the open art market. The chances of an eventual recovery therefore should be very good," said Robert Korzinek, art expert at specialist art insurer Hiscox.
The Picasso museum, housed in the 17th century Hotel Sale building in the Marais quarter in central Paris, is set to close this summer for at least two years for renovation work put at 20m euros.
The museum contains more than 250 paintings and 1,500 drawings by the Spanish artist.
Last month Picasso's portrait of a musketeer smoking a pipe sold for $14.6m in New York. Another painting, A Woman with a Hat, fetched $7.7m at the sale.
Picasso was born in Malaga, Spain, in 1881. He was a key force behind the Cubist movement but his work spanned a number of different and varied periods. He died in France in 1973.