A protester with trauma to the head was fighting for his life on Sunday after clashes with police during a demonstration over water storage facilities in France, a prosecutor said.

Organisers of the protest in the southwestern village of Sainte-Soline on Saturday had previously said one demonstrator was gravely wounded.

According to the latest figures from the prosecutor's office early afternoon on Sunday, seven protesters were injured, including three who had to be taken to hospital.

Twenty-nine policemen also sustained injuries, two of them badly enough that they had to be hospitalised.

Prosecutor Julien Wattebled said a 30-year-old man with a head trauma was fighting for his life after being among the three protesters admitted for emergency treatment.

A special inquiry had been opened "to determine the exact nature" of the injuries of these three people and "the circumstances in which" they received them, he said.

The other two badly injured demonstrators were a 19-year-old woman with a facial trauma and a 27-year-old man with a broken foot.

Campaigners in Sainte-Soline were trying to stop the construction of giant water "basins" to irrigate crops, which they say will distort access to water amid drought conditions.

Once they arrived at the construction site, which was defended by the police and gendarmes, clashes quickly broke out between the more radical activists and the security forces, AFP correspondents said.

Protesters threw projectiles including improvised explosives, while police responded with tear gas, water cannon and rubber bullets.

The clashes in Sainte-Soline came after days of unrest elsewhere in France over President Emmanuel Macron's pensions reform, which forced the cancellation of a visit by Britain's King Charles III.

Outrage over Macron imposing the bill without a parliamentary vote has sparked daily clashes between protesters and police in French cities in recent days.

But hundreds of thousands of French people have also since January peacefully marched against the reform, which includes raising the retirement age from 62 to 64.

Security forces have this week faced criticism for their heavy-handed tactics in dealing with the protests.

On Friday, the Council of Europe warned that sporadic violence in protests "cannot justify excessive use of force".