Police say three skinheads attacked the young man, one of them used a knuckle-duster to hit him and he fell down on the pavement.
The three skinheads, one of whom was a woman, escaped, according to police.
The incident took place not far from the Gare Saint Lazare at six o’clock on Wednesday evening, outside a building where a clothes sale was taking place.
The skinheads entered the building and exchanged insults with a different group of youths.
They then waited with re inforcements outside the building, where one of them hit the young man, whose name is Clément Méric.
He is originally from Brittany and was a student at the prestigious Sciences Po school of political science in Paris.
The biggest French student union, Unef, has called for a peaceful gathering at midday in front of the Sciences Po building, and another demonstration at 6.30 pm to call for the dissolution of all extreme right groups.
Unef issued a statement calling on interior minister Manual Valls to carry out his responsibilities in the face of a rise in attacks which were "racist, homophobic, xenophobic or deliberately aimed at those agitating for progress".
Valls has pledged to find the aggressors. He declared that the violence “bore the hallmarks of the far right and gravely endangered the Republican Pact.”
The far left Parti de Gauche says the attackers were from a group called the JNR (Young Revolutionary Nationalists). JNR leader Serge Ayoub denies that members of his group were involved.
Meanwhile Marine le Pen, leader of the far right Front National political party has condemned the attack as “obviously unacceptable, unbearable”.
She stated that her party “is in no way whatsoever linked with this attack”, and declared that she would take legal action against anyone who tried to suggest her party was involved.