The infamous Arbeit Macht Frei sign at the entrance to the Auschwitz Nazi death camp in Poland has been stolen.
The wrought iron sign was unscrewed and pulled down from its position above the gate overnight. Police are searching for the culprits.
Polish authorities have denounced the theft, while Israel's Holocaust museum branded it an "act of war".
More than a million people - 90% of them Jews - were murdered by the Nazis at Auschwitz during World War II.
Jarek Mensfelt from the Auschwitz-Birkenau Museum said staff were in shock over the theft.
"It is more than just stealing something. It is a desecration," Mr Mensfelt told the BBC.
He said that visitors came to Auschwitz to "pray, visit and remember" and there had been no threats against the place that he was aware of.
"Somebody who did this must have been a person who had a knowledge of our security system because all the area is closed at night and patrolled and there is a system of cameras.
"This was not an incident - this was a deliberate and organised action."
CCTV footage
Avner Shalev, director of Israel's Yad Vashem Holocaust Memorial, said the theft "constitutes a true declaration of war".
He added: "We don't know the identity of the perpetrators but I assume they are neo-Nazis."
Polish ex-President Lech Walesa described the theft as "unthinkable".
The BBC's Adam Easton in Warsaw says police are interviewing security guards and viewing closed circuit television footage.
It is not clear why it was stolen but museum officials say the widely recognised sign would be difficult to sell.
It is the first time the sign, made by Polish prisoners, has been stolen since it was erected in 1940.
The sign, which is occasionally removed by officials for conservation work, has been replaced by a replica.
During the Holocaust, hundreds of thousands of prisoners passed under the sign, whose words mean "Work Sets You Free", but the vast majority were murdered or worked to death.
The theft comes just days after the German government pledged 60m euros ($86m) to an endowment fund to help preserve the camp.
Auschwitz, which receives more than a million visitors a year, has been run as a state museum since 1947.
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Polish police said on Monday that they had found the infamous "Arbeit Macht Frei" sign that was stolen on Friday from the gate of the former Nazi death camp at Auschwitz.
Katarzyna Padlo, a police spokesman, said that police had also detained five young men and were planning to question them.
She added that the sign, which means "Work Sets You Free" and symbolises the cruelty of Nazi Germany, had been cut into three pieces.
Neo-Nazi groups were initially blamed for the act described by police as "professionally planned", and which happened at three o'clock on Friday morning.
Police said that once the criminals had removed the sign they carried it some 400 metres before taking it through a hole cut in a fence. After that it was, apparently, loaded onto a vehicle but investigators are puzzled as to just how the thieves managed to remove the 16-foot long and heavy cast-iron sign without anybody noticing.
"There was also a large horse in the room, taking up most of it."
It was the only way to transport the thing maybe? Either that or they were decided to destroy an enduring symbol of "the biggest lie in human history", since I'd be willing to put money on neo-Nazis being behind this.
Only two things are infinite - the universe and human stupidity, and I'm not sure about the universe: Albert Einstein.
