The Brazilian starlet said she had decided to speak out because she was frightened that "something could happen to me" and because she was "tired of living in a world of lies".
Miss Conceicao, who will give evidence in the trial on June 25, said that on the night of the encounter there had been intense competition between the showgirls to go to bed with the then prime minister in return for cash payments.
She claimed that an Italian glamour model named Barbara Guerra had entered the prime minister's bedroom first but had found him asleep.
As soon as she left the room, Miss El Mahroug dashed in and closed the door behind her. "She did everything in there," the Brazilian model said. "She later received an envelope with 5,000 euros in it."
Mr Berlusconi faces charges of abuse of office and paying for sex with the Moroccan-born Miss El Mahroug, whom prosecutors allege was working as an under age prostitute. He has denied all the charges.
His lawyers said Miss Conceicao's account was "totally unfounded".
The lawyers, Niccolo Ghedini and Piero Longo, said the false account was part of a concerted campaign against the former premier by a politically-biased press.
The trial resumed in Milan on Friday, with the first appearance by Miss El Mahroug's father.
He was asked to address one of the oddest elements of the whole saga – when Mr Berlusconi allegedly told police in Milan in May 2010 to release the teenage dancer on a theft charge.
He falsely claimed that she was the granddaughter of Hosni Mubarak, the then Egyptian president, and said that her detention could cause a diplomatic incident.
"I come from Morocco and Mubarak is Egyptian. They are two different countries," said Mohammed El Mahroug.
The court also heard from Ioana Visan, 24, a Romanian showgirl who attended the parties.
She was asked by a prosecutor whether she had ever had sex with the billionaire tycoon but refused to answer, saying they were "intimate matters".
But she said that she had received money from Mr Berlusconi and that he still pays the rent of the apartment that she lives in.
She denied reports that girls had performed stripteases in an underground "bunga bunga" room in Mr Berlusconi's villa, saying it was simply a type of home discotheque. "There was no eroticism or vulgarity," she insisted.
Barbara Faggioli, another showgirl who attended the soirées, described them as "quiet and enjoyable evenings with friends." She said the women referred to the payments they received from Mr Berlusconi as "pairs of shoes" – a form of code in which one pair of shoes was equal to 1,000 euros.
Miss Faggioli described Mr Berlusconi as a "father figure" and said he was codenamed "Betty" in telephone conversations between the showgirls.
She said a necklace that she was wearing in court was a gift from the media mogul, who had always treated her in a "gentlemanly" manner.
Mr Berlusconi risks being sent to prison for up to 15 years if convicted of the charges. The trial began in April 2011 and is expected to last many more months.