Rome (CNN) -- Allegations that Italian Prime Minister Silvio Berlusconi had sex with a teenage prostitute are "mud" thrown on him by political opponents who want to get rid of him, the scandal-prone politician said.
"But this time they've surpassed any limit," Berlusconi said in a statement. "The mud will fall on those who use justice as a political weapon."
The "judicial machination ... will not succeed in stopping us or in taking us away from our commitment to change the country. This time also they will not succeed," he said in the statement posted on his party's website Saturday.
Berlusconi is under investigation for allegedly having sex with the teenager, say prosecutors in Milan, who are linking the case to prostitution activity.
The teen, identified as nightclub dancer Karima El Mahrough and nicknamed Ruby, was 17 at the time of the alleged activity, from February till May.
She denied ever having sex with him in an interview recorded Saturday and broadcast Sunday.
She said Berlusconi was "a person who is accused of something he has never done.
"If you want to write, then write things that are true and not these lies and bar-room gossip," she told journalists and prosecutors. "If you don't like Berlusconi, then attack him with something else. Not with this taking advantage of a girl."
But she said she had received 7,000 euros (about $9,300) from him the first time they met, on Valentine's Day 2010, because a friend told Berlusconi she needed help. She said she was a guest at several dinners he gave, but that she did not know him well.
She told the prime minister, among others, that she was 24 "because I didn't want people to know that I was a minor," she said in the interview broadcast on Italy's Sky TG24.
The Milan prosecutor said Berlusconi is being probed for complicity in prostitution with a minor and abuse of power.
Italian media reports say Berlusconi has been summoned for questioning from January 21 to 23, but it is unclear if he will answer the prosecutor's summons.
The premier's attorneys called the investigation "absurd and groundless" and a "grave interference" in Berlusconi's private life.
Milan prosecutors started the investigation of this case in December after the premier called police and urged the teen's release from prison. She had been arrested in May on charges of theft.
Prosecutors also sent police to search the home of Nicole Minetti, a member of Berlusconi's party and a Lombardy regional council member.
Berlusconi allegedly asked Minetti to serve as Ruby's tutor so the girl would be able to leave prison.
Minetti is under investigation for allegedly favoring and aiding juvenile prostitution, prosecutors said. The teen is now 18 and is considered an adult.
The premier has always denied having any involvement with any prostitutes or helping prostitution or having wild parties at this house.
The latest investigation emerges after Italy's Constitutional Court struck down last week key parts of a law that would protect Berlusconi from prosecution.
The law was designed to halt criminal proceedings against top government officials for 18 months on the grounds that they are too busy to appear in court.
But Italy's top court ruled that judges, not politicians, should be the ones to determine if a defendant is free to appear in court.
Separate trials against Berlusconi are pending. He's accused of bribing a witness to commit perjury in one case and of tax fraud and other financial irregularities in another. Both trials began but were suspended when the immunity law was passed.
Berlusconi has called the charges politically motivated.
A third case against the prime minister, also related to tax fraud, is in preliminary stages.
Berlusconi has been tried on at least 17 charges since first taking office as prime minister in 1994, but he remains very popular with the Italian public.
CNN's Hada Messia contributed to this report.