Riot police have moved into the western French city of Nantes, clashing with hundreds of anarchists who broke shop windows, destroyed bus stops and pillaged the city center.

The regional prefecture says that six police officers were injured in confrontations on Saturday with up to 1,000 "radicals" who joined an estimated 20,000 people protesting plans to build an airport in the Loire-Atlantique region.

Four people were detained, the prefecture said.

Interior Minister Manuel Valls said the delinquents were from the "radicalized ultra-left" and were waging an "urban guerrilla" campaign. Valls said on iTele TV station that "these are individuals who are very violent."

There have been numerous, sometimes violent, demonstrations against building an airport in Notre Dame des Landes, a pet project of Prime Minister Jean-Marc Ayrault, a Nantes native.