Italy's prime minister has said that the magistrates who mounted what he called a politically motivated campaign to oust him should not go unpunished.
Silvio Berlusconi has been summoned for questioning over allegations that he paid an under-age prostitute.
In a 10-minute - often angry - TV address, Mr Berlusconi said the investigation was unconstitutional and procedurally flawed.
He vowed to pass new laws to prevent magistrates pursuing elected officials.
"There is nothing that I have to be ashamed of," Mr Berlusconi said.
"The government will continue to work, and parliament will make the necessary reforms to guarantee that a magistrate will not be able to try to illegitimately destroy someone who has been elected by the citizens."
'Just dinner guests'
The 74-year-old billionaire said he had refused to attend the questioning as he disputes the right of Milan magistrates to preside over the case.
"I can't present myself to public prosecutors that do not have either the functional or territorial competency, and also so as not to endorse this illegitimacy," he said, adding that this was the 28th time in 17 years that judges from the city had pursued him.
Mr Berlusconi, whose five-year term is due to run until 2013, accused the prosecutors of being politically motivated, launching the investigation just a week after he narrowly survived a confidence vote.
Milan prosecutors, he said, had used spying methods worthy of an anti-mafia investigation.
"One hundred and fifty agents mounted an operation against girls who were guilty of nothing more than being my dinner guests," he said in an address broadcast on Sky.
"They were taken for questioning, some without even being able to contact a lawyer, and kept there from eight in the morning to eight at night without being able to eat, without any contact with the outside world. Treated like criminals from some dangerous anti-mafia operation, a treatment utterly at odds with a state of law that cannot go unpunished," he said.
Much of the 385-page dossier detailing the investigation focuses on Karima El Mahroug, an 18-year-old Moroccan belly-dancer who attended Mr Berlusconi's parties when she was 17 and, prosecutors say, was paid to have sex with him.
Sex with a prostitute aged under 18 is an offence in Italy.
Mr Berlusconi quoted from public statements where she has denied having sexual relations with him, adding that she had been introduced to everyone as a 24-year-old.
But in a TV interview to be broadcast on a network owned by Mr Berlusconi's Mediaset group late on Wednesday, Ms Mahroug denied both having sexual relations with the prime minister, and media reports she was to receive a vast sum of hush money.
"He didn't lay a finger on me. I respect him as a person, and for helping me without asking for anything in return," she said, according to Italian newspaper Il Secolo XIX.