* Ex-premier asked for trial to be halted until after election
* Key witness "Ruby" makes brief appearance, no testimony (Updates with judges rejecting bid to suspend trial)
By Silvia Aloisi
MILAN, Jan 14 (Reuters) - Judges in Milan rejected a request by former Italian prime minister Silvio Berlusconi on Monday to halt his sex trial until after a national election next month, but allowed the key witness to avoid giving testimony in open court.
The three judges dismissed his lawyers' argument that his commitments as head of the centre-right coalition in the Feb. 24-25 election meant he would not have time to attend the trial.
Berlusconi is charged with paying for sex with a minor, and denies all charges. The trial's last session is currently scheduled for Feb. 4, meaning that a verdict could come before the election.
The nightclub dancer at the centre of the case, 20 year-old Moroccan Karima El Mahroug, more widely known under her stage name "Ruby the Heartstealer", made a brief appearance in court, wearing a white mini-skirt and looking relaxed and smiling.
She left shortly afterwards when Judge Giulia Turri said that statements already given to magistrates would be sufficient evidence and she would not have to testify in open court.
With the election campaign now under way, the hearing has revived memories of the "Bunga Bunga" sex scandal that hung over Berlusconi's last months in office before he resigned in Nov. 2011 at the height of the euro zone debt crisis.
The 76-year-old media billionaire is accused of paying for sex with Mahroug when she was under the age of 18, a crime in Italy, and abusing his office to have her released from police custody in a separate theft incident.
The former runaway is alleged to have been one of the main participants in a series of parties at Berlusconi's villa near Milan, where several young women have given accounts of lurid striptease shows.
Mahroug had been due to testify in December but failed to show up, telling her lawyer she was on holiday in Mexico. The prosecution alleged this was a ploy to try to delay a verdict.
The trial, in which dozens of aspiring show girls have described so-called "Bunga Bunga" parties at Berlusconi's residences, is the most sensational of his legal cases and has received huge media attention in Italy and abroad.
Niccolo Ghedini, one of Berlusconi's legal team, had argued that the election campaign and the accompanying interest meant the trial should be suspended.
But Turri and the other two judges rejected that argument, saying that involvement in political activity did not in itself constitute a reason to put a trial on hold.
"With this decision the court is intervening very heavily in the electoral campaign," Ghedini said after the decision.
"It's certain that there'll be a verdict before the elections; this is the intention of the magistrates."
Berlusconi himself was not present in court, although the judges rejected his lawyers' claim that he could not attend because he had to attend a political meeting in Rome. It was not clear whether he would be sanctioned over his absence.
Mahroug, who denies having slept with Berlusconi, appeared in the trial as a witness for the defence but Ghedini asked the judges to excuse her from testifying.
Berlusconi is leading his centre-right People of Freedom party (PDL) into the election, although it is still not clear if he will be running for prime minister.
Since entering the campaign in December, he has gradually improved his group's opinion poll ratings, but he is still trailing far behind the centre-left alliance that is expected to win.
Berlusconi's allies accused the Milan magistrates of trying to sabotage his election bid.
"The PDL is clearly bouncing back and magistrates are as usual entering the fray," said PDL deputy Enrico Costa.
Berlusconi could be sentenced to up to 15 years in prison but would not serve time unless he also lost the two appeals allowed by Italian law, usually a lengthy process. (Writing by Philip Pullella; Editing by Barry Moody and Kevin Liffey)