Projectile-throwing youths clashed with police today in two French cities on the sidelines of demonstrations honouring a protester who was killed during a similar confrontation over a dam project last week.
Riot police bearing shields fired tear-gas canisters in western Nantes to disperse masked protesters - some of whom lit fires, threw back the canisters and tore down street signs to use as projectiles.
Similar unrest broke out in south-western Toulouse, where authorities said some rioters among mostly peaceful protesters smashed bank windows and tore up public property.
The demonstrations testified to growing tensions between youths and police on the margins of mostly peaceful rallies by environmental activists in recent months.
A long-running stand-off has involved foes of an airport project in Nantes and, more recently, green groups have protested against the Sivens dam project in south-western France.
Today's rallies were called to honour Remi Fraisse, a 21-year-old protester who died last week in clashes between police and demonstrators near the south-western town of Lisle-sur-Tarn, not far from where the dam is to go up.
Such rallies - first primarily about environmental causes - have for some morphed into protests about alleged police brutality.
Hundreds of riot police and officers were mobilised in the two cities.
The local government office said at least two protesters were injured today in Nantes, as were two police officers - one burned by acid that was allegedly thrown by a rioter. At least 16 people were arrested.
In Toulouse, one police officer was slightly injured and eight demonstrators were detained, the local government office said in a statement.
Authorities denounced the violence and called for a return to calm.
French President Francois Hollande has called for an investigation into what caused Mr Fraisse's death and a post-mortem examination found that a large wound on his back was caused by some kind of explosion.