Fights first broke out on Wednesday night in the area surrounding the Capodichini airport in Naples, where seven arrests were made according to a web report in the Gazzetta dello Sport.
Two Bulgarians, said to be from clubs 'twinned' with Napoli's hardline 'ultra' supporter groups, were among those arrested.
The violence continued in the Piazza Garibaldi area of central Naples, where seven Poles were arrested following running battles with Napoli ultras during which iron bars and other weapons were used, said the report.
In a separate incident, four men from Naples were attacked by fans of the Polish first division club and their car was set on fire.
A report on Rainews.it said 65 Polish nationals were being held in Naples police cells while awaiting identity checks.
Police chiefs in Naples, meanwhile, have confirmed 14 officers suffered injury during the clashes.
Ultras from the Polish club are among the most feared in Europe, while ultras from Napoli are considered the most hardline in Italy.
In spite of their relative lack of success - Napoli have won only two league titles (1987, 1990) and one Uefa Cup (1989) - the 'Partenopei' are among Serie A's best supported clubs.
Rivalries among ultra fans in Italy now rarely make the headlines, although in 2014 that all changed when a Napoli fan, Ciro Esposito, died after being shot on the fringes of the Italian Cup final in Rome between Napoli and Fiorentina.
A Roma ultra, Daniele De Santis, was later charged and convicted of Esposito's death.
The Europa League, UEFA's second tier club competition, has given hardline fans around Europe the opportunity to spread their notoriety.
In February 2015, ultras from Dutch club Feyenoord ran riot in the Italian capital, causing thousands of euros worth of damages to famous city landmarks ahead of a Europa League fixture with Roma.
Earlier this season, Marseille hooligans ran riot in the Dutch city of Groningen ahead of the sides' Europa League clash.
Days before that fixture, Dutch police were on hand in Amsterdam to protect visiting Celtic fans from Ajax hooligans two years after fans of the Scottish champions were attacked in the city.