Eintracht Frankfurt fans clashed with police on Wednesday after arriving in Naples despite not having tickets for their team's Champions League decider with Napoli.
Footage broadcast by Italian media showed hundreds of supporters of the German club throwing smoke bombs and flares at police, who responded with tear gas.
A police car was also set on fire as chairs, bottles, and metal poles were thrown by hooded men dressed in black.
The mayor of Naples Gaetano Manfredi decried "urban warfare" and appealed for calm.
Italian media report that the Eintracht fans were moved from the city centre in buses around three hours from kick-off, while the atmosphere around the Stadio Maradona was calm.
According to Eintracht board member Philipp Reschke, the trouble began when a group of 150 locals attacked the German supporters while they were being escorted by police.
"It seemed a bit like the groups have searched and have found each other. The most important thing is: There are no injuries. Not even with the police," he told Bild newspaper.
"Tonight's violence must be condemned in the strongest possible terms. Perpetrators of violence and troublemakers destroy sport," tweeted German Interior Minister Nancy Faeser.
Supporters of the German club travelled to southern Italy even though Eintracht decided against selling tickets for the away section in Naples for the second leg of the last 16 tie.
Eintracht didn't take up their allocation after the Naples prefecture decided on Sunday to ban residents of Frankfurt from buying tickets for the match.
On Saturday, an initial ban on Eintracht fans, imposed on Friday by the Italian interior ministry which targeted all people residing in Germany, had been overturned.
Sunday's decision came after violence in the first leg won 2-0 by Napoli in Frankfurt, which led to nine people being taken into custody.
Eintracht fans have been under close surveillance by European governing body UEFA since the pitch invasion which greeted the club reaching the final of the Europa League, eventually won by beating Rangers.
They also clashed with Marseille fans in September, with both sets of supporters throwing smoke bombs at each other and one Eintracht fan being seriously injured.