A video depicting the last moments of an 18-year-old leftist who died during a scuffle with neo-Nazis in Paris suggests he was felled with a single punch.

French police reportedly get hold of CCTV footage from Paris transport company RATP showing Clement Meric, a student at Sciences Po University, in a fight in the central Rue de Caumartin earlier in June.

Meric is seen rushing towards his alleged killer, Esteban Morillo, to hit him in the back; Morillo turns and punches Meric in the face, RTL radio reported. Meric collapses to the floor almost immediately.

It is not clear from the images if Morillo, a young man in his 20s said to be close to far-right movements, wore a knuckle-duster and hit Meric a second time.

There is no evidence that the attack on Meric continued after he had been knocked to the ground, as was initially claimed.

The scuffle erupted as Meric, a campaigner with the Action Anti-Fasciste group, and three friends got involved in an altercation with neo-Nazis at a Fred Perry private sale in Paris's 9th district.

Early reports that Meric died because he hit his head on the pavement after being punched were dismissed by a postmortem examination.

Morillo and four far-right radicals were arrested. Morillo reportedly admitted throwing two punches but denied using a knuckle-duster. If the content of the video is confirmed he is likely to face manslaughter and not murder charges. The footage has not been released by police.

The incident caused a stir in France prompting calls for violent far right organisations to be banned.