AFP - French police have questioned a night watchman at a museum from where a valuable painting by 19th century artist Edgar Degas was stolen, a judicial official said Friday.

Police in the southern port city of Marseille released the man without charge on Friday after detaining him on Thursday evening, the official said, adding: "The investigation is continuing."

The picture, a colourful image of singers performing on a theatre stage, was missing when staff opened up the Cantini Museum in Marseille on Thursday.

The national museums service said the picture was a pastel work titled "The Chorus", worth 800,000 euros (1.14 million dollars) and measuring 32 centimetres by 27 (about 13 inches by 10).

It was loaned by the Orsay museum in Paris for an exhibition of theatre-themed artworks including some 20 works by Degas.

Prosecutor Jacques Dallest said on Thursday that the painting had been unscrewed from the wall, there was no sign of a break-in and the theft may have been an inside job.

A source close to the investigation said the watchman questioned was on duty early on Thursday morning when the theft was believed to have taken place.

The city hall, which runs the museum, said 70,000 people had visited the exhibition and the show was due to close in Marseille on January 3 and later go on tour to Italy and Canada.