The American actor, Robert De Niro, was arrested by the French vice squad at the Bristol Hotel in Paris yesterday and held for questioning in connection with an international prostitution ring.

Justice officials stressed that Mr de Niro is considered a potential witness at this point. He was detained on a warrant by Judge Frederic N'Guyen, who has been investigating the prostitution ring since October 1996. Police officers questioned Mr de Niro for several hours at his hotel just around the corner from the French Interior Ministry, before taking him to Judge N'Guyen's chambers at the Palais de Justice in the evening. He was released after nine hours.

The former Polish tennis champion Wojtek Fibak - a party friend of the 54-year-old star of Taxi Driver, Raging Bull, The Godfather and The Deer-Hunter - has already been placed under investigation for sexual assault in the same case.

Mr de Niro is in Paris for the making of Ronin, an action film by the director John Frankenheimer, about an American with a murky past. He is scheduled to remain until the end of March. Police had attempted to question him last Friday, but found he had already left the Bristol Hotel for the weekend.

Judge N'Guyen has also requested a meeting with Ms Brigitte Nielsen, the ex-wife of the actor Sylvester Stallone. The French television news station LCI reported that Ms Nielsen is suspected of arranging appointments between prostitutes and clients.

Capt Paul Baril, the former gendarme from the Elysee presidential palace who earned notoriety for framing three Irish people in the case of the "Irlandais de Vincennes", and for tapping telephones on behalf of the late President Mitterrand, has also been questioned regarding the Cote d'Azur-based network. LCI reported that Capt Baril is suspected of bringing Moroccan prostitutes to France.

A photographer who worked for soft porn magazines, Mr Jean-Pierre Bourgeois, a Swedish former model, Annika Brumarck, and a Lebanese businessman, Mr Nazihabdulatif el-Ladki, have been placed under investigation for procuring. It is Mr el-Ladki who apparently provided details of the ring to Judge N'Guyen.

Some 40 prostitutes were reportedly matched up with wealthy customers as far away as New York and the Arab Gulf states. Clients included men involved in arms sales and the jewellery business, and the prostitutes received between £600 and £2,400 per meeting, plus travel expenses.