Local newspaper Le Telegramme has published pictures of groups of so-called "anti-rioters" who wore hoods and masks as they apprehended and beat up suspected troublemakers in the city on Friday night.

The Ouest France newspaper also published an interview with a 25-year-old man who said he was a member of the armed forces and intervened to support the police along with around 30 colleagues so as to "not leave the country to burn".

The Forfusco navy unit based in Lorient has "opened an inquiry which is underway. Until the results are known, there will be no further comment," the defence ministry said in a statement to AFP.

Lorient mayor Fabrice Loher told AFP he was unable to confirm what had happened, but said he had "seen people in masks. We thought they were rioters".

"What is important to me is what the Forfusco says," the mayor said, adding that he was worried about the impact of the incident on his town's reputation.

More than 3,500 people were arrested over the last week during France's worst urban violence since 2005. It was sparked by the fatal shooting of a teenager of North African origin by police during a traffic stop in western Paris.

The unrest spread from poor, multi-ethnic areas of Paris to dozens of other localities where police were sometimes overwhelmed by mobile gangs of young people who set fires and looted shops.

The police shooting and riots have led to a bitter debate about racism in the security forces and the role of immigration and poverty in the breakdown in law and order.

Lorient prosecutor Stephane Kellenberger said Wednesday that no investigation was underway in the absence of any legal complaint or "concrete or objective element" to justify it.

Four people were arrested by police in Lorient on Friday night.

An unnamed police officer quoted by Le Telegramme at the weekend said officers initially let the "anti-rioters" intervene "because it was helping us" before they realised "they were going at it a bit too strongly".

The paper quoted a witness saying the men described themselves as "patriots".

Forfusco is composed of marine riflemen and special forces commandos, according to the defence ministry website.