A Burnley fan was stabbed in the leg and four others were injured in clashes before Thursday’s 3-1 Europa League playoff defeat by Olympiakos in Athens, the Premier League club said.
The BBC reported that 23 Burnley fans were arrested for being drunk and disorderly.
The club said the majority of their 900 travelling supporters were “impeccably behaved”.
“However, five of our supporters were unfortunately injured – including a person who received a stab wound to the leg – and received medical attention at the stadium.
“Those injured all travelled independently to the stadium, outside the organised travel operation,” Burnley said.
The defeat was Burnley’s first of the Europa qualifying campaign.
Chris Wood’s first-half penalty cancelled out Kostas Fortounis’ opener before Andreas Bouchalakis put the hosts ahead after the break and Fortounis scored his second.