Vandals defaced a monument to victims of a World War Two pogrom against Jews in Poland, covering it with racist inscriptions and swastikas in green paint, police said on Thursday.
The attack, condemned by the Polish government and a leading Jewish group, was the latest in a series of racist and xenophobic acts of vandalism targeting the small Jewish and Muslim communities in eastern Poland as well as the tiny Lithuanian minority.
At least 340 Jews were burnt alive by their Polish neighbours in a barn in the 1941 pogrom in the eastern town of Jedwabne. The site was later turned into a memorial.
On Wednesday a police patrol ran into the damaged site. We immediately started an investigation, said Andrzej Baranowski, police spokesman in the nearby city of Bialystok.
Vandals also smeared a wall surrounding the memorial with signs saying I'm not sorry for Jedwabne and They were highly flammable. They obscured the Hebrew and Polish signs on the memorial itself with paint.
This is a perfect example of vandalism and stupidity, but we don't know the exact motives yet, Baranowski added.
All the recent anti-Semitic and xenophobic incidents were probably perpetrated by the same people, Poland's interior ministry said this week, and they are all under investigation by the Bialystok police.
Foreign Minister Radoslaw Sikorski condemned the act of vandalism as alien to Polish tradition.
In Polish society there can be no place for allowing such acts, even if they involve only small groups of extremists... I am convinced that those responsible will soon be found and will suffer the legal consequences of their actions, he said in a statement on the ministry website.
The American Gathering of Holocaust Survivors and their Descendants said the attack, which came on the eve of the anniversary of Nazi Germany's invasion of Poland on Sept 1, 1939, was aimed not only against the Jewish community but the reputation of Poland.
In erecting the Jedwabne memorial Poland demonstrated its determination to confront its anti-Semitic past; by its swift reaction to these vandals it will demonstrate its commitment to fight hate in the present, Elan Steinberg, the group's vice-president, said in a statement.
A 2001 Polish investigation concluded that the Jedwabne pogrom was inspired by Poland's then-Nazi occupiers and the case remains a traumatic memory for Jews and many Poles today.
Poland was home to Europe's largest Jewish population of some 2.5 million until World War Two, when most of its Jewish citizens perished in the Nazi-sponsored Holocaust.
The few who survived the war faced periodic oppression by the communist regime installed in Poland after 1945.
Poland is a largely homogenous Roman Catholic country but religious and ethnic minorities are more common in eastern regions near the borders with Belarus and Ukraine.