One protestor said: "We want justice. It's not true that our murdered friends sold drugs or were mobsters."
Police had said that they suspected the shootings late on Thursday to be a fallout from a drug-related turf war.
The six men were killed outside a tailor's shop by six assailants who fired 130 bullets, allegedly using Kalashnikov rifles and smaller weapons.
The victims were from Ghana, Togo and Liberia and were aged between 25 and 31.
A seventh man, from Ghana, was being treated for injuries.
The police said that the attackers may have been linked to the Italian mafia, and may have killed the men for attempting to enter into the drug trade.
Sandro De Franciscis, president of the Caserta province which includes Castelvolturno, said: "The arrogance of the Camorra [the Italian mafia] has reached intolerable levels."