French Socialist government leaders and Green deputies are at odds after the death of a protester during clashes with paramilitary police at the site of a controversial dam project in south-west France.

Rémi Fraisse, a 21-year-old student, died instantly on Saturday night after being hit in the back by a grenade, according to the results of an autopsy. However, the conclusions of further analyses, which should indicate whether the weapon was fired by gendarmes or by protesters, are not expected until later on Tuesday.

Protesters opposed to the Sivens dam project in the Tarn region say it will destroy a reservoir of biodiversity and will benefit only a small number of farmers. Those promoting the project say the dam is in the public interest as it will ensure irrigation and the development of high-value crops.

Breaking a two-day official silence since the weekend violence, the interior minister, Bernard Cazeneuve, on Tuesday condemned Green party deputies who had accused the security forces of responsibility for Fraisse’s death before knowing the investigation results.

In veiled remarks targeting Green deputies, who have been vociferous in their criticism of the police action since Sunday, he denounced their “shameless politicisation” of the weekend events.

President François Hollande announced on Tuesday that he had phoned Fraisse’s father – who said on Monday that he would press murder charges – to express his condolences and “solidarity”.

Speaking after protesters rallied in several French cities to denounce “police repression” on Monday evening, Hollande guaranteed that “the whole truth” would be uncovered. However, he added that “excessive public statements” were not helping to calm things down. Eight people were taken into custody during a protest by about 600 people in Nantes, near where protesters had occupied the proposed site of a new airport. Those plans have been suspended.

The Greens, who left the government in April and who are now openly hostile to its policies, have been critical of the police presence at the site of the future Sivens dam, where environmental protesters have been camping out. However, the gendarmes say that legitimate demonstrators were accompanied by provocateurs from the Black bloc, who were part of a small group of violent protesters throwing missiles at the security forces.

The former housing minister, Cécile Duflot, gave a radio interview early on Tuesday in which the Green deputy called Fraisse’s death “an indelible stain” on the government’s record. She condemned the fact that two days after the incidents “not a single member of the government” had offered condolences to the family.

Her comments seem to have stung the government into its orchestrated response, which coupled calls for calm with implicit criticism of the Greens, who rejected the official response as “scandalous”.

Tree-clearing for the dam at Sivens, about 30 miles from Toulouse, began last month as part of a project that has been planned for more than 20 years.

On Monday, an independent experts’ report gave succour to the environmental campaigners by pointing out that the proposed dam on the Tescou river was too large for the purposes of irrigating about 20 farms, and that ecological concerns had been neglected.