OSWIECIM, Poland - Polish authorities stepped up security checks at airports and border crossings and searched scrap-metal yards yesterday as the search intensified for the infamous Nazi sign stolen from the Auschwitz death-camp memorial.

The brazen predawn theft Friday of one of the Holocaust's most chilling and notorious symbols sparked outrage from around the world, and Polish leaders have declared recovering the 16-foot sign a national priority.

The sign bearing the German words "Arbeit Macht Frei" - "work makes you free" - spanned the main entrance to the Auschwitz death camp, where more than one million people, mostly Jews, were killed during World War II.

The grim Nazi slogan was so counter to the actual function of the camp that it has been etched into history. The phrase "Arbeit Macht Frei" appeared at the entrances of other Nazi camps, but the long, curving sign at Auschwitz was the best known.

Police deployed 50 officers, including 20 detectives, and a search dog to the Auschwitz grounds, where barracks, watchtowers, and rows of barbed wire stand as testament to the Nazi atrocities.

Spokeswoman Katarzyna Padlo said police had questioned all security guards at the site and searched local scrap-metal businesses, while Dariusz Nowak, a police spokesman in Krakow, said investigators were working around the clock on the case.

The director of the Auschwitz-Birkenau memorial museum, visibly shaken by the theft, said he believed it was carried out by professionals.

"I think it was done by specialists," Piotr Cywinski said. "It was a very well-prepared action."

British historian Andrew Roberts said the sign would generate huge interest on the burgeoning market for Nazi memorabilia.

Security guards patrol the 940-acre site, but they pass by any one area only at intervals. Cywinski said that gave thieves between 20 to 30 minutes to remove the sign and carry it off.

Museum spokesman Pawel Sawicki said the sign was made of hollow steel pipes and was believed to weigh only around 65 to 90 pounds. "A single person could lift it," Sawicki said.

A replica of the sign, produced when the original underwent restoration work years ago, was quickly hung in its place Friday.

Michael Pick, 47, a history teacher from Brisbane, Australia, was glad the museum had put up a replica.

"The irony of the saying is something that we talk about in the classroom," he said, standing amid snow and below-freezing temperatures. "It would be better if it [the sign] were authentic, but I would be incredibly disappointed if I showed up today and there was nothing there."

The Simon Wiesenthal Center, a leading Jewish human-rights group, urged Poland to intensify its investigation and bring the thieves to justice.

"The fact is that the 'Arbeit Macht Frei' sign has become the defining symbol of the Holocaust," said Rabbi Marvin Hier, the center's founder and dean, "because everyone knew that this was not a place where work makes you free, but it was the place where millions of men, women, and children were brought for one purpose only - to be murdered."