According to television images, the words "Jews out" were sprayed on the grave in a Warsaw cemetery.
A spokesman for the police said an investigation had been launched to try to identify the perpetrator.
Sendler died in May 2008 at the age of 98. Before World War II, she helped poor Jewish families in Warsaw, where she worked as a social worker.
In autumn 1940, she took considerable risks to bring food, clothing and medicine to the inhabitants of the ghetto.
In 1942, she was smuggled children out of the ghetto to Catholic institutions and convents. The children were hidden in suitcases carried by firefighters or garbage bins or hidden under the coats of people entitled to enter the ghetto.
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