Hundreds of thousands of people passed under the sign to their deaths
Polish police are using a sniffer dog in their search for thieves who took the infamous Arbeit Macht Frei sign over the entrance to Auschwitz.
Detectives and other police officers were sent to the site of the former Nazi camp where more than one million people, mostly Jewish, were murdered.
A replica of the sign, which translates as "Work Sets You Free", has been put up temporarily.
The theft on Friday has caused outrage in Israel, Poland and elsewhere.
"The theft of such a symbolic object is an attack on the memory of the Holocaust, and an escalation from those elements that would like to return us to darker days," said Avner Shalev from the Yad Vashem memorial centre in Jerusalem.
"I call on all enlightened forces in the world who fight against anti-Semitism, racism, xenophobia and the hatred of the other to join together to combat these trends."
In Brussels, European Parliament President Jerzy Buzek, a former Polish prime minister, appealed to the thieves to return the sign.
"Give it back out of respect for the suffering of over a million victims, murdered in this Nazi camp, the biggest cemetery of humankind," he said.
Metal bars cut
The wrought-iron sign was half-unscrewed, half-torn off from above the death camp's gate between 0330 and 0500 on Friday, said a police spokeswoman.
HISTORY OF AUSCHWITZ SIGN
Made by Polish political prisoners in 1940
Letter 'B' thought to have been reversed as act of defiance - making it appear upside-down
Locals say Red Army soldiers were bribed to leave it in Poland after camp was liberated
Occasionally replaced by replica while conservation work is done
Investigators say at least two people would have been needed to steal the 5-m (16-ft), 40-kg (90-lb) sign.
Police said they were reviewing footage from a surveillance camera that overlooks the entrance gate and the road beyond but declined to say whether the crime was recorded, The Associated Press news agency reports.
Auschwitz museum spokesman Jaroslaw Mensfelt said it might have been too dark for the camera to have captured images.
He said the thieves apparently carried the sign 300m (yards) to an opening in a concrete wall. The opening was left intentionally to preserve a poplar tree dating back to the time of World War II.
Four metal bars that had blocked the opening had been cut and footprints in the snow led from the wall opening to the nearby road, where police presume the sign was loaded on to a vehicle.
Another police official said earlier that while all leads were being considered, police were 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.
A 5,000-zloty ($1,700; £1,050) reward has been offered for information leading to the capture of the thieves.
Auschwitz, which receives more than a million visitors a year, has been run as a state museum since 1947.
Germany recently pledged 60m euros ($86m) to an endowment fund to help preserve the camp.