A youth slapped former prime minister Manuel Valls when he was on an election tour in France's Brittany region and was wrestled to the ground by a security guard, video showed.

Valls, 54, was walking past a group of people in the town of Lamballe after coming out of its municipal offices. He appeared to first shake hands with the youth before the latter shouted: "This is Bretagne!" and reached out to slap the ex-premier.

As Valls recoiled, a security guard seized the youth in a choke-hold, pushed him back against a fence and then forced him to the ground, a video aired by local newspaper Le Telegramme showed. Police said later the 18-year-old man was in custody.

There is an extreme right-wing local political group in Brittany, a region in France's far northwest, but it was unclear if the detained man was politically motivated, a local prosecutor said.

Valls, a Socialist who is running in the Left's primaries on Sunday to be a candidate for president, did not appear to have been hurt in the incident. He continued shaking hands, telling the crowd, "it's nothing," another video showed.

Valls told local reporters later: "I have never been afraid to have contact with the French people. Democracy can never mean violence.