French police have launched an investigation after football fans unfurled an offensive banner at the League Cup final on Sunday.
The match was between Paris St Germain and Lens, from the industrial north-east of France.
Paris fans opened out a banner which described people from northern France as unemployed, inbred paedophiles.
President Nicolas Sarkozy, who was at the match at the Stade de France, said the behaviour was "inadmissible".
If caught, those responsible could be charged with inciting hatred or violence, and could face jail sentences.
The 25m wide banner was unfurled briefly during the second half of the match between Paris St Germain and Lens.
The wording on the banner is a play on the title of a recent hit French film, Welcome to the land of the Ch'tis, which portrays French northerners as warm and big-hearted.
President Sarkozy, who was there with his son, is reported to have immediately asked the president of the French football league to have it taken down.
In a statement from the Elysee, the president said he was "a dismayed witness to the unfurling of a hateful banner."
Both football clubs have filed legal complaints, as have the French Football League.
"Pieces of the banner have been found and officers from the technical and scientific police are examining them to see if any DNA traces can be found," said interior ministry spokesman Gerard Gachet.
The incident has also been condemned by the mayor of Paris, Bertrand Delanoe, and members of parliament.
Correspondents say Paris St Germain have had trouble with their supporters in the past - they have been involved in clashes with rival fans which have had racist undertones.