A senior Manchester police chief says it would be "naive" to think Wednesday's Europa League final in Stockholm will be trouble free.
Riot police needed to intervene to keep Manchester United and Ajax fans apart when they met in Amsterdam in 2012.
Ticketless fans are expected in Sweden, having missed out on both club's official allocations of 9,500.
"You would be naive to think you are not going to get anything," said chief superintendent John O'Hare.
"It is a final, in Europe, between two really big, established, well-supported teams. There will always be a minority of individuals who want to use this as cover to cause trouble," the Greater Manchester Police officer added.