Irena Sendler, 98, a Polish Catholic social worker who helped lead a smuggling operation that rescued thousands of children from Warsaw's Jewish ghetto during World War II, died May 12 at a hospital in that city. She had pneumonia.

After the German invasion of Poland in 1939, an estimated 400,000 Jews were forced into the ghetto, the size of 16 city blocks. They faced disease, execution or deportation to concentration camps, and the hopelessness led to a ghetto revolt in early 1943 that ended in a slaughter by the Nazis.

Mrs. Sendler, aiding Jews since the start of the war, became an early activist in the clandestine group Zegota. The underground movement -- whose members faced execution if caught -- formed in 1942 after the deportation of 280,000 of Warsaw's Jews to the Treblinka death camp.

Mrs. Sendler used her senior position with the city's welfare department to win access to and from the ghetto and set up a network of 25 associates to help organize escapes and falsify documents.