Thousands of supporters marched in grief and anger Saturday to honor an anti-fascist activist who died after a brawl with far-right militants, while authorities opened a murder investigation against a 20-year-old skinhead suspected of delivering the fatal blow in a killing that has shocked France.

France’s Socialist government also took a first step toward banning the security branch of a nationalist youth group that the suspect and four alleged accomplices had claimed ties with, according to the Paris prosecutor.

The death of 18-year-old Clement Meric, a student at Paris’ prestigious Sciences-Po political science university, has renewed concerns that hate groups are on the rise — not just in France, but across Europe.

A medical examiner determined that Meric died from head trauma sustained in the fight that erupted after a chance encounter Wednesday between the far-right militants and anti-fascist activists including Meric in a posh Paris shopping district, prosecutor Francois Molins said at a news conference. He said a murder investigation was under way into one suspect — a security guard who was identified only as “Esteban” — while he and three other skinheads were also facing charges for group violence in the fight that led to Meric’s death.

The four suspects were being held, and a fifth suspect, a 32-year-old woman named Katya who was said to be Esteban’s girlfriend, was facing the prospect of preliminary charges for complicity in group violence, Molins said. The suspects, under police questioning, acknowledged links to an ultranationalist group known as “Troisieme Voie” — or Third Way, he said. None of the suspects had a prior criminal record, though Esteban was known to police for possession of banned weapons in Paris in May 2011, the prosecutor said.

Prime Minister Jean-Marc Ayrault’s office said in a statement Saturday that he had asked the interior minister to immediately take steps, under a 77-year-old domestic security law, toward dissolving a far-right group known as Revolutionary Nationalist Youth — considered the security unit of Third Way.

Militant extreme-right groups have become increasingly visible in France, and the government said after Meric’s death that it wants to ban fascist and neo-Nazi groups. Extreme-right groups have gained attention in numerous European countries, particularly Greece, where the Golden Dawn party, broadly vilified for alleged Nazi sympathies and violence against immigrants, holds seats in parliament. Last month, the World Jewish Congress said it’s greatly concerned about the emergence of what it called neo-Nazi parties in places like Greece, Hungary and Germany.

On Saturday, demonstrators poured into the streets of eastern Paris to honor Meric, chanting “we don’t forgive, we don’t forget” and marching behind a banner that said he was “forever in our memories, forever in our hearts.”