PARIS -- A gang of more than 20 youths -- thought to be North African immigrants -- terrorized hundreds of train passengers in a rampage of violence, robbery and sexual assault on New Year's Day, French officials said yesterday. The five-hour-long criminal frenzy was "totally unacceptable," French President Jacques Chirac told reporters. "Those guilty will be found and punished, as they deserve." The gang of between 20 and 30 youths boarded the train, heading from Nice on the French Riviera to Lyon, in eastern France, early on Jan. 1, as it carried 600 passengers home from New Year's Eve partying overnight. Once inside, they went wild, forcing passengers to hand over mobile phones and wallets, and slashing seats and breaking windows. A 20-year-old woman cornered by several of the marauders was sexually molested. "It was a real scene of pillage on the train," said the regional state prosecutor, Dominique Luigi, adding that the passengers were in a state of "panic." Train staff alerted police, and the train pulled into a station to wait. The three officers who initially turned up later were joined by reinforcements. A waitress in a bar near the station said two young women from the train had come inside in tears. "They told me there had been groping. They talked about sexual assaults. They were really traumatized," she said. The train resumed its journey with the heavy police presence on board but, just before Marseille, the youths pulled the emergency stop and many escaped. Only three -- two 19-year-old Moroccans and a minor, all living in France -- were arrested. Both men were being held for robbery and one also was facing charges of sexual assault. The minor was to be judged separately. Three others -- a man and two boys -- were arrested briefly in Marseille but were released despite reports they were carrying a knife, a screwdriver and a small amount of hashish.