French police are holding in custody six supporters of Paris Saint-Germain, prosecutors said, suspecting them of "looking for a scrap" with fans of Newcastle ahead of Tuesday's Champions League clash.

"Six people were arrested among several dozen, wearing black and with their faces masked, who were heading towards a bar where there were English fans," Paris prosecutors said.

A police source said four of the people arrested were in the French state's "S file" of people believed to pose a national security threat, adding they appeared to be "looking for a scrap" with the Newcastle crowd.

Prosecutors said the grounds for holding those arrested were "participation in a group formed with an aim to commit violence or destroy or damage property," adding that "investigations are ongoing."

France has been on alert for public violence in recent weeks, after the deadly stabbing of a teacher in northern city Arras by a former pupil last month and another knife killing of a 16-year-old in southeastern France last week.

Violent gatherings organised by the far right over the weekend were broken up by police. (AFP)