(CNN) -- A sign synonymous with the Nazi work camps of World War II was stolen overnight from the Auschwitz Concentration Camp memorial in Poland, police said Friday.

Police were "alerted at 5 a.m. local time on Friday by museum guards" that the infamous sign reading "Arbeit Macht Frei" -- "Work Sets You Free" in German -- was stolen, according to police spokeswoman Agnieszka Szczygiel.

The heavy iron sign "was removed by being unscrewed on one side and pulled off on the other," Szczygiel said. "It is also believed that this was a planned event and that several people were involved as the sign was at remarkable height."

Police have launched an investigation.

The chairman of Yad Vashem, Israel's Holocaust museum, called the theft shocking.

"While we don't yet know exactly who stole the sign, 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.

"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," he said.

More than 1 million people died in the Auschwitz-Birkenau concentration camp complex; about 90 percent of the victims were Jews.

Rabbi Marvin Hier, founder and dean of the Los Angeles-based Simon Wiesenthal Center, called the sign "the defining symbol of the Holocaust" and urged authorities to intensify their investigation.

"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," Hier said. "The audacity and boldness of this crime deserves the full attention of the Polish government."

The center calls itself one of the largest international Jewish human rights organizations.

CNN's Laura Perez Maestro contributed to this report.