Eintracht Frankfurt fans clashed with police after arriving in Naples despite being banned from Wednesday’s Champions League last-16 match with Napoli.
Away fans were barred from the second leg after incidents at their first meeting last month.
But a police car was set on fire by a group of hundreds of supporters and smoke bombs and flares thrown at officers, who responded with tear gas.
Local media said Eintracht fans were also attacked by Napoli ultras.
It said the incidents had caused “terror among passers-by and shopkeepers”.
Footage shared on social media showed chairs and flares being thrown at a small number of police dressed in riot gear.
Italy’s deputy prime minister Matteo Salvini wrote on Instagram: “These are not fans, they are criminals, thugs. I wonder if they would make the same mess in Germany?”
Eintracht board member Philipp Reschke said trouble began when a group of 150 locals attacked away supporters while they were being escorted by police.
“It seemed a bit like the groups have searched and have found each other,” he told the German newspaper Bild. “The most important thing is there are no injuries, not even with the police.”
German interior minister Nancy Faeser said: “Tonight’s violence must be condemned in the strongest possible terms. Perpetrators of violence and troublemakers destroy sport.”
The match was played without incident inside the ground with Napoli winning 3-0 to seal progression to the quarter-finals.
Uefa charged Eintracht Frankfurt with racist behaviour, lighting of fireworks, throwing of objects and acts of damage following a Champions League tie in Marseille in September, and nine people were taken into custody after the first leg against Napoli.