Away fans were barred from the second leg after incidents in the first leg 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?”
The match begins at 20:00 GMT on Wednesday with Napoli leading 2-0.
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.
Source – BBC Sport
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