Hundreds of passengers were terrorised by a mob of youths rampaging through a train on the French Riviera One young woman was sexually assaulted, travellers were robbed of phones and cash, and carriages were wrecked in the two-hour assault, it emerged yesterday.

Up to 40 youths involved in the attacks had taken advantage of a special fare of about 80p to travel from Marseilles and Avignon to celebrate the New Year in Nice.

They were part of a larger group of 100, many drunk from their overnight festivities and escorted by police to Nice station for their return journey at dawn on Sunday.

A small police team accompanied the Lyons-bound train at first. But trouble broke out soon after their departure.

Eventually the driver stopped the train at Les Arc-sur-Argens to appeal for extra police assistance. Only three officers turned up and violence continued as they awaited reinforcements before boarding the train. The line was blocked for an hour and a half as police gained control of the train.

Passengers spoke of being warned they would be killed if they refused to hand over belongings or told police what had happened.

They said they were too frightened to intervene and several refused to lodge complaints of theft or assault.

"It was a real scene of pillage on the train," the regional prosecutor, Dominique Luiggi, said yesterday as details emerged. "Passengers were in a state of panic."

One witness said there had already been assaults and threats, including other indecent attacks on young women, on Saturday's outward journey.

He said the robbers were of Arab origin and had boasted about their plans to cause more trouble on the train.

President Jacques Chirac promised yesterday to bring the gang to justice. He said those responsible would be "found and punished".

France's Le Parisien newspaper likened the youths to the gang in the film Clockwork Orange.