Eintracht Frankfurt supporters clashed with police on the streets of Naples after travelling for Wednesday's Champions League fixture against Napoli despite a ban on visiting fans being in place.
Videos on social media showed crowds throwing missiles at officers wielding riot shields, while a police car appeared to be engulfed in flames.
Italy's deputy prime minister Matteo Salvini responded to one video of the clashes on Instagram, writing: "These are not fans, they are criminals and delinquents.
"I wonder if they would make the same mess in Germany. Maximum solidarity and support to the police forces."
The Italian Ministry of the Interior barred Eintracht supporters from the match after crowd trouble overshadowed last month's last-16 first leg in Germany – a 2-0 win for Napoli.
Eintracht attempted to reverse that measure, but they ultimately opted to forgo their away allocation after the Administrative Court of Campania upheld a decree barring Frankfurt residents from purchasing tickets.
However, widespread reports in the build-up to Wednesday's game said some visiting supporters were in the city, while there were local reports suggesting Napoli ultras had also become involved.
Eintracht were previously charged by UEFA with racist behaviour, lighting of fireworks, throwing of objects and acts of damage after crowd trouble also marred their Champions League fixture at Marseille in September.