Dozens of rioters smashed windows, flipped cars and threw rocks at police, calling for justice and accusing law enforcement of racism for assuming the victims were drug traffickers.
Television footage showed young men wielding metal bars halting traffic and making motorists leave their vehicles.
"We want justice. It’s not true that our murdered friends sold drugs or were mobsters," one protester told reporters.
Police say at least six hitmen sprayed 130 bullets at the men late on Thursday in Castelvolturno, a town of 20,000 northwest of Naples, apparently using a Kalashnikov automatic rifle as well as smaller weapons.
It was one of the bloodiest shootouts in recent memory blamed on the Camorra, the Naples version of the mafia, and stunned a region long used to violence from organised crime.
The six dead were from Ghana, Togo and Liberia, and between the ages of 25 and 31. A man from Ghana was wounded in the shooting, Italian media reported.
"The arrogance of the Camorra has reached intolerable levels," said Sandro De Franciscis, president of the Caserta province which includes Castelvolturno.
Cardinal Crescenzio Sepe, archbishop of Naples, called for an end to the violence.
"Put down your guns. What you use to kill people today will kill you and your families tomorrow," Sepe told reporters. "As long as these killers are not defeated we will always have a cemetery filled up by hate and with violence."