Hebrew graffiti thanking the Nazi leader Adolf Hitler for the Holocaust and denouncing Zionism was sprayed inside the Yad Vashem Holocaust museum in Jerusalem, an AFP correspondent said on Monday.
The main suspects are a small but active fringe group of extremist ultra-Orthodox Jews who are known for their vitriolic opposition to the existence of the modern state of Israel.
Seven giant slogans, including one which read: "Thank you Hitler for your wonderful Holocaust that you arranged for us, it's only because of you that we got a state at the UN," were sprayed in Warsaw Ghetto Square near the sculpture depicting the Warsaw Ghetto uprising.
More graffiti was sprayed next to the cattle car memorial, which remembers how millions of Jews were transported from all over Europe to the Nazi death camps.
Other slogans read: "The Zionist leadership wanted the Holocaust," and: "If Hitler hadn't existed, the Zionists would have invented him."
Another said: "The war of the Zionist regime is not the war of the Jewish people."
The red, white and black graffiti was written both in formal Hebrew characters and in handwriting, and signed "The global cynical mafia."
Speaking to Israel's army radio, Yad Vashem director Avner Shalev said there was no doubt the vandalism was carried out by ultra-Orthodox extremists.
"One of the slogans was signed with the words 'World Haredi Jewry'," he said, using the Hebrew word for ultra-Orthodox Jews.
"This time, a red line has been crossed," he said, indicating that police were examining CCTV footage from cameras which are positioned around the site.
Police spokesperson Micky Rosenfeld confirmed basic details of the incident, and said an investigation had been opened.
Education Minister Gideon Saar condemned the vandalism and expressed confidence the police would catch the perpetrators.
"Whoever desecrated and soiled Yad Vashem with these disturbed slogans did it with the aim of harming public sensibilities," he said in a statement.
There are several ultra-Orthodox groups which do not believe that a Jewish state should exist without the appearance of the Messiah, the best-known of which is Neturei Karta, whose several hundred members live in Mea Shearim, a crowded ultra-Orthodox neighbourhood of Jerusalem.
Yad Vashem is the main museum in Israel commemorating the six million Jews who were killed by the Nazi regime during World War II.