Twitter user @Rajigolo shared this picture of a smashed window at the Piebury Corner shop near Arsenal's Emirates Stadium.
Piebury Corner's website shows the shop's facade before the damage:
The Metropolitan Police described the incident as "football-related".
A spokesman said a man had been arrested in the area for a public order offence, unconnected to the damage at Piebury Corner.
A man in his 40s was taken to Whittington Hospital in north London with cuts to his head.
A London Ambulance Service spokesman said the man's injuries were not thought to be serious.
Paul Campbell, owner of the Piebury Corner pie shop, said the attack was "completely unprovoked".
A pie shop near Arsenal's stadium was allegedly attacked by rival Italian supporters before this evening's Champions League match.
Piebury Corner, near the Emirates Stadium in north London, has had to close after dozens of thugs smashed windows, breaking chairs and swinging belts.
Witness Emily Gould, who works at the restaurant, said around 50 customers were inside the premises when a 30-strong mob of rival supporters stormed the building prior to kick-off.
She said: "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."
She said one man had to be taken to hospital with what she believed was a deep gash to the head after he was struck by a swinging belt buckle.
Napoli have been charged over alleged match-fixing in a game against Sampdoria on the final day of the 2009-10 season, the Italian football federation prosecutor said in a statement.
The club, their former third choice goalkeeper Matteo Gianello and ex-footballer Silvio Giusti will go before the federation's disciplinary committee after information was passed from a criminal investigation in Naples.
Sampdoria won the game and entered the preliminary stage of the Champions League.
The development is part of a wide-ranging criminal and sporting probe into illegal betting and match-fixing in Italy which have already led to several arrests and bans.