Thousands of supporters marched in grief and anger Saturday to honor an anti-fascist activist who died after a brawl with far-right militants, as authorities opened a manslaughter probe against a 20-year-old man suspected of delivering the fatal blow.

France's Socialist government also took a first step toward banning a far right 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 far-right groups are on the rise in France and 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 an upscale Paris shopping district, prosecutor Francois Molins said at a news conference. He said he had sought a murder investigation concerning one suspect -- a security guard who was identified only as Esteban -- and charges of group violence against Esteban and three other men over the fight that led to Meric's death.

However, late Saturday, an investigating judge rejected Molins' effort to pursue a murder investigation, and instead filed preliminary charges against Esteban for "deadly blows" -- which amounts to violence leading to death without intention to kill, said prosecutors' office spokeswoman Agnes Thibault-Lecuivre. If convicted Esteban faces up to 15 years in prison.

A fifth suspect, a 32-year-old woman named Katya, was handed preliminary charges for complicity in group violence, she said.

Molins said the suspects acknowledged links to an ultranationalist group known as "Troisieme Voie" -- or Third Way, Molins 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, he said.

Militant extreme-right groups have become increasingly visible in France, and the government said after Meric's death that it wants to ban them. 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."

The fight after which Meric died erupted outside a clothing store where members of the two groups had run into each other by chance Wednesday, Molins said, citing witness accounts and testimony by the suspects during police questioning. The suspects said they have responded to alleged provocation by a small group that included Meric, he said.

During questioning, "the one named Esteban acknowledged to police that he had struck Clement Meric twice -- bare-fisted, he claimed -- including the blow that caused him to fall to the ground," Molins said. "A friend of Clement Meric said he saw him (Esteban) with brass knuckles, while another witness of the scene referred to a `shiny object' in his hands."

A police sweep of Esteban's home turned up two sets of brass knuckles -- though it was not immediately clear if they had been used in the fight, the prosecutor said. Early results of the autopsy showed that Meric had died of skull and brain trauma, and a hemorrhage near the brain following "several blows," Molins said.