Milan, Italy (CNN) -- Italian Prime Minister Silvio Berlusconi will be tried on charges of sex with an underage prostitute and abuse of power, an Italian judge ruled Tuesday.
Judge Cristina di Censo decided the trial would begin April 6, Italian judicial authorities confirmed.
Three judges will preside over the criminal trial in Milan. All of them are women.
Prosecutors in Milan filed the request for trial last week.
Berlusconi denies the charges, and his lawyers have argued that the Milan courts do not have the authority to try a prime minister or jurisdiction over the case because of where the alleged crimes were committed.
Milan chief prosecutor Edmundo Bruti Liberati requested the fast-track trial on charges that Berlusconi paid for sex with nightclub dancer Karima El Mahrough, who was 17 at the time of the alleged activity, and later intervened with the police after she was arrested on an unrelated charge.
An associate of Berlusconi's denied that the prime minister had wild parties at his house or that she had procured girls for him to have sex with.
"Absolutely yes, I would deny that. Absolutely yes," regional lawmaker Nicole Minetti told CNN.
Prosecutors have tried to link Minetti to the allegations against Berlusconi. She has not been charged with a crime.
She said there were "sometimes pretty girls" at Berlusconi's parties, but that the dinners were not the wild bacchanals depicted in media reports about the prime minister.
"He sings. He tells stories, any type of stories, I mean even personal stories, of his experience of politics, humorous stories, that is a little bit of how the evenings were," she said. "I mean, nothing lurid. That is how it was."
Berlusconi dismissed the accusations as "groundless" when the prosecution request was filed last week, saying the call for a trial was a "farce."
There was no abuse of power, he said.
And he said he was "sorry because these (allegations) have offended the dignity of the country. They've thrown mud on the government, on the country and on myself at an international level."
Berlusconi, a wealthy media magnate turned politician, has faced trials in the past, but this is the first time he has been charged with abuse of power as prime minister or personal misconduct.
He has previously had to defend himself against charges of bribery and financial misconduct.
Berlusconi has been tried on at least 17 charges since he was first elected prime minister in 1994, but none of the cases have resulted in lasting convictions; several were overturned on appeal.
He has always denied any wrongdoing, and had previously claimed immunity from prosecution, though a 2008 law granting him immunity was overturned the following year.
Thousands of Italians took to the streets in some 200 cities across the country Sunday to protest Berlusconi's alleged behavior toward women.
The largest gathering was in Rome, where organizers said 100,000 people gathered, under the slogan, "If not now, when?"
In Milan, organizers estimated that 60,000 people demonstrated.
Protests also took place outside Italy, including marches in Tokyo and Geneva, Switzerland.
Sunday's demonstrations were the biggest anti-Berlusconi rallies since the most recent sex scandal broke.
Berlusconi has denied that he has ever paid anyone for sex.
The investigation began in December, several months after Berlusconi called police to urge them to release El Mahrough, nicknamed Ruby, from jail, where she was being held on theft charges.
Both El Mahrough, now 18, and Berlusconi have denied they ever had sex.
El Mahrough said she did not know Berlusconi well but that she did receive 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.
The young woman's former roommate told investigators that El Mahrough confided to her that she did have a sexual relationship with the premier.
Berlusconi's party argued that he believed that Ruby was then-Egyptian President Hosni Mubarak's niece and the phone call to the police station on her behalf was done to avoid a possible diplomatic crisis with Egypt.
The lower house of Italy's Parliament voted against allowing Milan prosecutors to search property belonging to Berlusconi as part of the investigation.
CNN's Dan Rivers and Hada Messia contributed to this report.