Twenty West Ham fans were arrested and six injured in a night of clashes with rival fans in Italy, police said.
The supporters were in Sicily for the return leg of the club's UEFA Cup clash with Palermo on .
Tension was already high after T-shirts bearing the slogan 'Hammers versus The Mafia' - a reference to Sicily's links with the Italian criminal underworld - were sold outside Upton Park before the first leg earlier this month, which the Hammers lost 1-0.
Trouble flared just before midnight and continued for more than an hour as rival groups threw bottles and chairs at each other in the city's Teatro Massimo district.
Palermo police spokesman Virgilio Alberelli said 500 people were involved in the brawl and when officers tried to break it up they also came under missile attack.
Some 20 Britons and a local were arrested for violence against public officials while 109 other West Ham fans were stopped, identified and released.
It took police in riot gear more than an hour to bring the situation under control.
Six officers, six English people and five locals suffered minor injuries.
More than 2,500 West Ham fans have travelled to Palermo for the game.
A police spokesman said extra security would be in place on Thursday night.
He added: 'We had been alert already ahead of the match and now we will be increasing security.
'The clashes were in the city centre between two groups of fans and left the streets covered in glass.
'Bottles, cans, glasses and chairs were thrown by both groups - it was a huge battle which took more than an hour to control.
'We would appeal for both sets of fans to keep calm and not react to provocation from opposition fans.'