Fan violence erupted in Naples on Wednesday ahead of a Champions League match between Napoli and Eintracht Frankfurt from which visiting German fans have officially been banned.
Videos on social media showed police officers being attacked by troublemakers with tables, chairs and pyrotechnics in central locations of the city, and at least one police car was set ablaze.
Citing police sources, the ANSA news agency said that ultras and hooligans from the Frankfurt and Napoli camps were engaged in the riots.
Frankfurt board member Philipp Reschke said before kick-off they were told by police that "Napoli fans attacked our fans which set into motion what we have seen.
"It is clear that no one wants to see these scenes. It was the riots we feared from the day of the draw onwards. We must come to terms with it but that will take time.
"The most important thing is that no one was injured, neither on the police side not among the fan groups."
Frankfurt residents were banned from attending the last-16 return leg by the prefecture of Naples in connection with security concerns after fan violence around the first-leg match in Germany last month. The Frankfurt club, which had initially won an appeal against a first fan ban imposed by Italy's interior ministry, waived the ticket contingent it was entitled to and urged fans not to travel to Italy.
"You have seen that such decrees are useless. You can't prevent riots with them," Reschke said.
Several hundred fans of the Europa League champions still made their way to Naples and marched through the city on Wednesday, first peacefully before trouble started at dusk.
Police eventually managed to put the Frankfurt ultras into buses in which they brought back to their hotel in the port area. The buses were reportedly attacked by Napoli fans with stones and bottles.
"The scenes of destruction of our historic city centre are unacceptable," Naples mayor Gaetano Manfredi said.
"We condemn in the strongest possible terms these unspeakable acts by those responsible for them, from whichever side they originated."
Frankfurt fans had started arriving on Tuesday night, according to Italian reports joined by ultras from Napoli's Italian rivals Atalanta. Every fan was registered by police.
Buses bringing the fans to their hotel were attacked with pyrotechnics, reportedly set off by Napoli ultras. A large police presence was seen in front of the hotel.
ANSA said that a security force of 800 has been deployed to maintain order around the match.
In a video, Napoli captain Giovanni Di Lorenzo, whose team won the first game 2-0, has appealed to his own side's fans to refrain from violence and not to be provoked.
UEFA president Aleksander Ceferin has said that clubs should not be allowed to host European matches in their stadium in the future if visiting fans are not permitted to attend.
