They assert that Mr. Berlusconi paid for sex with Karima el-Mahroug, a Moroccan-born nightclub dancer nicknamed Ruby Heart-Stealer, before she turned 18, and that he called police to help release her from custody after she was detained for theft in Milan in May.

On Wednesday, Mr. Berlusconi said he had called police to intervene on Ms. Mahroug’s behalf because he had been told that she was the niece of President Hosni Mubarak of Egypt. “I intervened as prime minister because I was concerned that there might be an international diplomatic incident,” Mr. Berlusconi said, adding that he “always” helped “people in difficulty.”

Ms. Mahroug has said that she did not have sex with the prime minister but that he did pay her 7,000 euros, or about $9,500, the first time she attended a party at his villa outside Milan last spring. Paying for sex with a minor under 18 is illegal in Italy.

Although the scandal has been raging in the Italian news media for weeks and unsettling a growing number of Italians, it has not translated into political defeat for the prime minister, who holds a narrow majority in Parliament. The center-left opposition is weak, and Mr. Berlusconi’s center-right coalition has shown no clear intention of replacing him.