France's radical left is under pressure after the killing of a student in Lyon was blamed on "anti-fascist" militants.

Quentin Deranque, a 23-year-old maths student and nationalist activist, died on Saturday, two days after being beaten up on the street by a group of young men.

State prosecutor Thierry Dran told a news conference on Monday afternoon in Lyon that a murder investigation has been opened into the death.

He said that Deranque was kicked and punched by "at least six" individuals, and that the autopsy showed fatal damage to his skull and brain.

On Thursday afternoon Deranque had given support to a hard-right feminist collective Némésis, which staged a small protest against a left-wing politician's visit to Lyon's Institute of Political Studies (IEP), commonly known as Sciences-Po.

Mobile phone video footage taken a few streets away from the protest's location showed three young men being kicked and punched by a larger group.One person - believed to be Quentin - remained lying on the ground.

According to Dran, there was an initial confrontation between two groups. A short time later Deranque was one of three nationalist activists who, having failed to get away, were cornered by the opposing group.

Ministers from President Macron's centre-right government blamed "far-left" militants for the killing.

Interior Minister Laurent Nuñez said: "Manifestly it was the far-left that was at work … This was a mob-killing, with blows that the perpetrators probably did not intend to be fatal."

According to Justice Minister Gérald Darmanin, "it is the far-left that killed him. Of that there is no question."

Dran said investigations were continuing to identify the suspects but no arrests had yet been made.

In a tense political atmosphere ahead of important municipal elections next month, all eyes on Monday were on the radical left party France Unbowed (LFI) and a banned security outfit La Jeune Garde (Young Guard) that in the past helped provide protection at its rallies.

Members from the Némésis collective – whose demonstration Thursday was against a meeting given by LFI euro-deputy Rima Hassan - said they recognised activists from La Jeune Garde in the group that initially disrupted their protest. The group has denied involvement.

Two of the seven Némésis protesters were attacked and hurt in that first confrontation, Dran said.

Hassan on Friday condemned the incident and called for an investigation. In a post on X, she said LFI provided the only security protection for her during the visit and that they were "in no way" involved in the violence.

The Deranque family's lawyer, Fabrice Rajon, said Deranque "was not a security agent or the member of any kind of protection group.

"He had never been involved in trouble … He defended his convictions in a non-violent way."

Friends told French media that Deranque had rediscovered his Catholic faith in recent years and was active in nationalist politics.

Alice Cordier, of Némésis, said Deranque was one of "15 or so young boys" who the collective asked to provide protection at their demonstration.

"It wasn't a professional security service, just friends who came to protect the girls."

Dran told the news conference that after the beating Deranque was able to start walking home with one of his friends. But en route it became clear that he was in a deteriorating medical state, and the emergency services were called.

Right-wing politicians were outspoken in their denunciation of LFI, saying it had a clear responsibility for encouraging violence by so-called "antifa" - anti-fascist militants.

Euro-deputy Marion Maréchal - niece of National Rally leader Marine Le Pen - said: "The militia of [LFI leader Jean-Luc] Mélenchon has killed and has blood on its hands."

According to Laurent Wauquiez, parliamentary leader of the centre-right Republicans: "Quentin fell beneath the blows of monsters guided by hate and blinded by violence."

Bruno Retailleau, who last week declared himself a candidate for the Republicans in next year's presidential elections, said: "Jean-Luc Mélenchon … contrary to what you claim, it is not the police that kills in France, it is the far-left."

On Sunday Mélenchon said his party had "nothing to do with this story. Those who accuse us are committing a libel.

"We express our consternation, but also our empathy and compassion for [Deranque's] family and friends. We have said dozens of times that we oppose all forms of violence."