ROME – Italian Prime Minister Silvio Berlusconi will go on trial in April over allegations that he paid a 17-year-old girl for sex and used his status to spring her from police custody, a judge ruled Tuesday.

Judge Cristina Di Censo scheduled the first hearing for April 6, in a move that brings long-running allegations about the 74-year-old premier's scandalous private life to a head and severely dents his hold on power.

Following the announcement, the prime minister's camp reiterated its claims that Berlusconi was the victim of a witch hunt by his political opponents.

"We didn't expect anything else," his lawyers said.

Berlusconi, whose bedroom antics sparked a nationwide protest by Italian women on the weekend, will be tried by three female judges, prompting the popular magazine Famiglia Cristiana to quip the premier has met his "nemesis."

In nearly two decades in politics, the billionaire tycoon has been dogged by a long list of investigations -- he faces two corruption trials later this year -- but it is the sex scandals that have caused most outrage.

In the trial, the prime minister will have to answer allegations that he paid for sex in his sprawling luxury mansion outside Milan with an erotic nightclub dancer called "Ruby the Heart Stealer," who was 17 at the time.

He is also accused of persuading police to release Ruby after she was arrested for alleged theft and hand her over to another alluring young woman, Nicole Minetti -- his former dental hygienist, now a senior regional official.

Berlusconi's defence claim he did not abuse his power because he mistakenly believed Ruby to be the niece of the Egyptian President Hosni Mubarak, and had been dutifully helping a fellow leader avoid diplomatic fallout.

Prosecutors allege he was in fact trying to keep his secret liaison with the young Moroccan out of the public eye.

Berlusconi faces three years in prison if found guilty of paying a minor for sex, and from six to 12 years for abuse of power.

But a lawyer close to the case said Berlusconi's defence will try to cast doubt on whether the court in Milan should be handling the case at all.

Berlusconi could also seek a vote in parliament to strengthen his position and argue that his office prevents him from attending hearings.

Even if convicted, the prime minister is unlikely to ever serve time in prison since sentencing guidelines in Italy are lenient for over-70s.

Berlusconi has denied ever paying for sex, let alone with Ruby, whose real name is Karima El Mahroug and who only turned 18 in November 2010.

While using the services of prostitutes is not a crime in Italy, paying for sex with a girl under the age of 18 is illegal.

Italy's Justice Minister Angelino Alfano said the prime minister was "presumed innocent" until proven guilty and should not resign.

"The left-wing, beaten in the elections and in parliament, is trying to use justice" against Berlusconi, said Daniele Capezzone, spokesman for Berlusconi's ruling People of Freedom party.

"They won't succeed," he added.

But Franco Pavoncello, a professor of political science at Rome's John Cabot University, said Tuesday's legal move "not only weakens him, but also weakens the willingness of his allies to follow him."

"If we're not in the final act of this government, we're at least in the penultimate one," he said.

It is not the first time the premier has been mired in a sex scandal. In May 2009, Berlusconi was accused of frequenting another 17-year old, Noemi Letizia, who called him "papi", provoking his wife to file for divorce.

The public slanging match between Berlusconi, the opposition and magistrates over the allegations has caused alarm among business leaders and unions, who said the government should be concentrating on Italy's real problems.

"This country has a right to ask that the huge economic crisis be taken care of, rather than continuing to deal with the private rooms of the premier," said Susanna Camusso, the head of Italy's main union CGIL, after the ruling.

According to a poll published on Monday, Berlusconi's popularity rating collapsed to 30.4 percent in February partly under the weight of sex scandals but also because of Italy's economic woes.

Gross domestic product rose just 0.1 percent in the fourth quarter compared to the third, according to official data released on Tuesday -- after growth of 0.3 percent in the third quarter and 0.5 percent in the fourth.