French police clashed for a second day running Saturday with protesters squatting on protected swampland the government wants to clear for an airport, a battle that has strained the ruling Socialists' ties with their Green allies.

Government ministers and later the office of Prime Minister Jean-Marc Ayrault meanwhile redoubled their efforts to appease the protesters and take the heat out of the confrontation.

At least seven people were injured during clashes Saturday.

Riot police fired tear gas and squatters threw stones and glass bottles as thousands of people demonstrated in the commune of Notre-Dames-des-Landes near the western city of Nantes:

As well as the clashes at the the 1,650-hectare (4,000-acre) site of the proposed airport, police and demonstrators faced off in Nantes itself.

Among those hurt was a police officer knocked out by a piece of paving stone thrown by a protester in Nantes, local officials said.

Police detained nine people at the airport site, seven of whom were later released. The other two were held for possession of a weapon.

The planned airport north of Nantes, which is scheduled to replace the city's current airport in 2017, is a pet project of Socialist Prime Minister Jean-Marc Ayrault, who was the city's mayor from 1989 until this year.

He says it is a key component of his plans for economic regeneration for the region.

But the Greens and other politicians have denounced the plan for environmental reasons, while on the ground, eco-warrior protesters have set up camps around the area.

The movement has pitted the Socialist government against its environmentalist parliamentary allies, who have called for President Francois Hollande

A day after Interior Minister Manuel Valls took a tough line against the protesters on Friday, there were signs that the government was trying to de-escalate the confrontation.

On Saturday, three ministers -- of agriculture, ecology and transport -- released a joint statement reasserting the "necessity" of building the airport.

At the same time, they vowed to strengthen their "intiatives towards respecting biodiversity and preserving agricultural land", according to the statement by Delphine Batho, Frederic Cuvillier and Stephane Le Foll.

The ministers added that the area would not be cleared until a scientific committee had given the green light.

Later Saturday, the prime minister's office announced it would set a "committee of dialogue" next week to hear all sides of the debate.

It was acting it said acting "for the sake of appeasement."

On Saturday, police said that 3,200 people demonstrated in Nantes, while protest organisers estimated the turnout at 8,000.

At both sites, protesters booed both Ayrault and Valls. On Friday, the interior minister had referred to the protest camps as a "cyst" that should not be allowed to grow.

The 580-million-euro ($747-million) project was approved in 2008, with construction expected to start in 2014.

The airport is set to have an initial annual capacity of four million passengers, and supporters say it will provide a major boost to tourism in western France.

The crackdown at the site began Friday, when police detained three people amid clashes that wounded two officers, a demonstrator and two television journalists as angry protesters hurled petrol bombs, bottles, stones and firecrackers at police.

Officials said two of those detained were later released.

On Saturday, some 40 police trucks were parked in four columns in the middle of the proposed airport site, where about 500 squatters have set up camp.

Police were clearing the road to the area after protesters blocked it to barricade an impromptu village they set up on November 17 during an initial rally.