Witnesses described how around 30 men, described as fans of the Italian club, had team scarves tied round their faces and started shouting in Italian at those inside the busy restaurant.
One worker at the Piebury Corner eatery described how they had to duck for cover and how one customer had been attacked with a belt leaving "blood all over the floor".
The London Ambulance Service confirmed a man in his 40s was taken to hospital with head injuries, although they were not thought to be serious.
Emily Gould, who works at the Piebury Corner on Holloway Road, said around 50 customers were inside the restaurant when the supporters stormed the building.
She told Sky Sports News: "I think they were Italian men with Napoli colours and scarves tied around their faces.
"They were shouting something in Italian, chanting and causing trouble down the street.
"Then they came in to the shop. I was terrified. They started swinging belts around.
"The customers were very frightened."
Piebury Corner owner Paul Campbell said the attack was "completely unprovoked". The restaurant has had to close due to the damage caused.
He said: "We have had Napoli fans inside all day today, they have been having a good time and not bothering anybody at all. They have been no problem.
"Then we had a minority of them come in, being violent, picking up chairs and tables.
"There were three girls sat near me and these thugs just came in and grabbed the chairs from under the girls.
"It was completely unprovoked, I think they just picked us at random because they saw a busy restaurant and they wanted to fight someone - anyone."
Mr Campbell said he had no reason to suspect any trouble from Napoli fans before kick-off.
He said: "We are popular with home fans and the travelling European fans too. This just came from nowhere."
15 pubs full of blokes up & down Holloway Road and the Napoli fans choose to attack... A Pie shop. I think that sums that up perfectly. Cowards- Clockendgooner