By Natalia Ojewska

WARSAW—The head of Poland’s Supreme Court on Wednesday defied a judicial overhaul that has put the nationalist government on a collision course with the European Union, showing up to work surrounded by cheering supporters despite a new law forcing her ouster.

The court’s chief justice, Malgorzata Gersdorf, refused to follow a government order calling for the retirement of 27 of the court’s 82 judges, herself included. Justice Gersdorf told cheering supporters on the steps of the courthouse that she would fight to protect the…