Many people passed under the sign to their deaths
The infamous Arbeit Macht Frei sign at the entrance to the Auschwitz Nazi death camp in Poland has been stolen.
The wrought iron sign, whose words mean "Work Sets You Free", was unscrewed and pulled down from its position above the gate in the early hours of Friday.
Polish authorities 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.
Investigators say at least two people would have been needed to steal the five-metre-long (16ft), 40kg (90lb) sign.
The theft occurred between 0330 and 0500 local time on Friday, police spokeswoman Katarzyna Padlo told AP news agency.
Jarek Mensfelt, from the Auschwitz-Birkenau Museum, told the BBC: "It is more than just stealing something. It is a desecration.
"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," he added.
"This was not an incident - this was a deliberate and organised action."
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", while Poland's chief rabbi said he could not imagine who would do such a thing.
"If they are pranksters, they'd have to be sick," said Michael Schudrich.
Polish President Lech Kaczynski called on the public to help recover the sign, which he described as a "worldwide symbol of the cynicism of Hitler's executioners and the martyrdom of their victims".
Israeli President Shimon Peres also condemned the theft during a special meeting with Poland's Prime Minister Donald Tusk.
In a statement, his office said Mr Peres "expressed the deepest shock of Israel's citizens and the Jewish community across the world", and urged Poland to "make every effort" it could to find the criminals and return the sign.
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.
Oswiecim Police spokeswoman Malgorzata Jurecka told AFP: "All leads are being considered, but we are focusing on a theft ordered by a private collector or a group of individuals."
It is the first time the sign, made by Polish prisoners, has been stolen since it was erected in 1940.
It has now been replaced with a replica while the hunt continues.
A 5,000-zloty ($1,700; £1,050) reward has been offered for information leading to the capture of the thieves, reports AP.
The cynical slogan "Arbeit Macht Frei" was also used at the entrances to other German Nazi camps, including Dachau and Sachsenhausen, although the one at Auschwitz is perhaps the best known.
Hundreds of thousands of prisoners passed under the sign into the camp during the Holocaust, but the 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.
Here is a selection of comments BBC News readers have sent to us in reaction to this story:
This act is simply repulsive. I am left speechless and saddened. Please let me know if there is a fund that has been set up that will allow myself and others to contribute to the reward being offered.
Brian, East Lyme, Connecticut, USA
How is this different to people removing metal plaques from war memorials or lead off church roofs? It may not be sold as a sign, but with metal prices as high as they are it could already be melted down and reused. As a sign it may have sentimental meaning, but it could just be an act of theft, not "a declaration of war" against the Jewish state.
Phil, London, UK
The sign belongs at the bottom of the ocean, or melted down in a forge. The thieves were probably sons or grandsons of survivors, putting that nazi propaganda to rest.
Kyle W Lewis, USA
I am appalled at Israel's reaction to the theft of the sign. Their reaction suggests they are paranoid and still see hatred in others' actions regardless of whether they were directed at them. The sign on Auschwitz's entrance is an iconic sign and there are many people who would steal such a piece of history. It shows paranoia on Israel's part to brand this an act of hatred toward Jews. I believe the taking of the sign is more likely to be a bit of fun, an opportunity for someone to get their hands on a piece of history and get one over on those charged to protect the site.
Rebecca Bradbury, Exeter, UK
Abominable behaviour. I have visited the camp and it is the most incredibly atmospheric and moving place I have ever been to. You can just feel the despair and depravity. Why would anyone wish to add insult to the memory of these people.
Denise Carey, Northampton, UK
After visiting Auschwitz in November for the first time, I think it is awful to see what some people can stoop to. It's such an historic, sad and overwhelming site, how anyone can do this is unthinkable and I hope they are caught and severely punished. Horrible people.
Chris Carless, Liverpool, England
I recently visited Auschwitz and the sign was the first symbol that ingrained itself on my memory. The message on the sign displayed the Nazis' sickening subterfuge for callousness and cruelty and mass murder. The whole experience was very emotional and it beggars belief that anyone could go to such a place with ideas of theft or to dishonour the memories of the poor souls who suffered there. An absolute henious act. Shame on you.
Michael Tremarco, Liverpool, UK
Very shocking. The cynical sign reminded everyone how sick the human mind can become. Unfortunately, there is no shortage of maniacal and murderous people to take the place of the departed despots and their depraved followers.
Freddie Chisale, Norton, Zimbabwe
A Nazi slogan, intended to enforce and celebrate the enslavement of their Jewish victims, is taken away. And the theft is condemned by Jewish and Israeli leaders as an anti-semitic act. History moves in mysterious and ironic ways.
Stephen Engelhard, London, UK
It is absolutely disgusting and we are utterly appalled by such a vicious act of vandalism.
Rachel Melieres-Frost, Petersfield, UK
I am absolutely appalled to think somebody would do this. I visited Auschwitz last year and I regard this sign as a symbol of Nazi atrocities. I hope they recover the sign soon.
Mike Phillip, Axbridge, Somerset, UK
I am so sad that the Jewish people and countless others who were victims of the atrocities by the Nazis at Auschwitz must endure this blatant act of terrorism. At this time of year we often see acts of humanity to renew our faith in man, but this inhumanity has broken my heart.
Heidi Zehner, Orlando, USA