Authorities have said finding the stolen sign is an "absolute priority" for the police, who are stepping up border security in their efforts to catch the thieves.
Polish police say they have tightened security checks at airports and border crossings, and have begun rummaging through scrap metal yards to intensify their search for the stolen sign at Auschwitz.
Authorities have also placed road blocks in the southern region and mobilized about 40 forensic experts to gather evidence on the museum grounds, said regional police spokesman Dariusz Nowak.
"This is an absolute priority for the Polish police," said national police spokeswoman Grazyna Puchalska.
The Auschwitz museum has offered a 100,000-zloty reward (24,000 euros or $34,000) for information leading to the discovery of the sign. Polish police have offered an additional 5,000 zlotys.
First serious theft at Auschwitz
The thieves removed the iron sign by unscrewing it on one side of the gate and pulling it off the other side.
Police have said they do not have any leads on the identity of the thieves but they are questioning museum guards and checking security camera footage.
Museum spokesman Jaroslaw Mensfelt said it was the first serious case of theft at Auschwitz.
"This was done by someone who knew what they wanted to do," Mensfelt told Polish news agency PAP. "They had to know how to enter the museum grounds, how to remove the sign and the routes the guards take."
A copy of the sign has now been placed on the entrance gate.
Shock in Poland
The theft of the historically significant sign caused shock in Poland. Polish Senate Speaker Bogdan Borusewicz told Polish radio that the incident was "deplorable and embarrassing".
"I hope the police recover the sign soon and that it won't have been cut into pieces," Borusewicz said.
Deputy foreign minister Andrzej Kremer told the PAP news agency that he was shocked and said the sign was a "key symbol" of Auschwitz.
Former Polish President and former leader of the communist-era opposition movement Solidarity told Polish television that the theft was "unthinkable" but said he did not believe it was an ideological act.
"It's a criminal act," he said. "It can't be comprehended any other way."
Outrage in Israel
There has also been a strong reaction in Israel to the incident. The head of Jerusalem's Yad Vashem Holocaust memorial, Avner Shalev, called the theft a "declaration of war."
"We don't know the identity of the perpetrators but I assume they are neo-Nazis," Shalev said in a statement.
"I am certain the Polish government will do everything possible to track down those criminals and put them on trial," he added.
Editor: Andreas Illmer
A huge security operation has been mounted in Berlin as the city welcomes US President Barack Obama. It's his first visit to the German capital since initially taking office in 2009.
For the first time in Germany, a Muslim community has been granted 'corporation under public law' status. Abdullah Uwe Wagishauser, its chairman, tells DW that this includes rights as well as responsibilities.
In summer, charcoal fumes and the spit and crackle of grilled meat are as much a part of Berlin as the Berghain club or the Brandenburg Gate. DW’s Jefferson Chase investigates Berlin’s smoldering BBQ culture.