Football fans in Italy were filmed throwing furniture and flares at riot police ahead of a match between Napoli and Eintracht Frankfurt.
The Champions League draw in Naples is due to start in around two and a half hours, but Italian media said the German ultras had already damaged a number of police vehicles.
Some were set on fire, according to photos taken at the scene.
Videos shared on social media show people wearing dark jackets, some wearing motorcycle helmets and balaclavas, charging towards officers as they brandish riot shields.
One of the clips was shared by Match of the Day presenter Gary Lineker, who described the scenes as “horrific”.
The message he retweeted estimated that there were around 300 “hooligans” holding the city “hostage” and facing a team of only around 20 police officers.
The tweet, by Italian sports journalist Tancredi Palmeri, also says the attackers are made up of Eintracht and Atalanta fans from Bergamo in the north of the country.
The Sportitalia television channel tweeted that there was “total destruction and guerrilla warfare” in Piazza del Gesù, one of the most important squares in central Naples.
Tonight’s game at the Stadio Diego Armando Maradona is the second leg of the draw, with Napoli winning the opener 2-0.
Italy’s interior ministry initially banned Eintracht Frankfurt fans from buying a ticket to the match due to fears of potential violence.
However, the sports news site Soccer Italy said Atalanta fans may have bought tickets for some of them, allowing them to circumvent the rule.
The ministry’s decision came after three Napoli supporters were injured in violence ahead of the first leg on February 21, according to the Daily mail.
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