Loïk Le Priol, a former soldier who reportedly has links to the far right, told Hungarian police in Záhony he was going to Ukraine to fight.
Three combat knives were found in his vehicle, said police in a statement.
His name has been on a French police watchlist since the deadly shooting of Federico Martin Aramburu on Saturday.
Mr Aramburu, a winger with 22 Argentine caps, had come to Paris with clients of his travel agency to watch the Six Nations France-England rugby match.
