A policeman was taken to hospital after he was attacked by fans at Middlesbrough's 8-1 demolition of Manchester City.
Violence flared between City supporters and the police during a pitch invasion at the Riverside Stadium.
Several officers were injured as they were targeted by angry fans.
A Cleveland Police spokesman said the trouble flared after a City fan grabbed a four foot long metal pole used to keep fans off the pitch.
Match Commander Supt Martin Campbell said: 'After a full debrief and having spoken to a club steward, it appears that towards the end of the game a visiting fan removed a 4ft long metal safety barrier which was in place to prevent access to the pitch.
'Fearing that it could be used as a weapon or to gain access on to the playing area, an officer removed this barrier and a skirmish then took place, spreading into other areas of the crowd.
'At this point officers began to come under attack as seats were pulled off and a number of police officers were struck by seats, one fully in the face.
'Police reinforcements were immediately called into the ground.
'Other officers suffered minor injuries, with only one requiring hospital treatment for an injury to his hand.
'I would like to praise the match referee who did an excellent job; it seems he was fully aware of the unfolding events and he ensured that players were swiftly removed from the pitch at the end of the game.
'Middlesbrough supporters too were extremely well natured and I'd like to praise them too. It is a shame that such a fantastic end to the season and what was always intended to be a great family occasion was marred by the behaviour of a small minority.'
Seven visiting fans were arrested for public order offences.
King of the Kippax fanzine editor Dave Wallace said: 'I was at the Riverside but I left when the sixth goal went in so I did not see the violence, but from what I heard it kicked off after a policeman grabbed a lad who had been shoved on to the pitch.
'After that some of the other City supporters joined in. They were trying to help him.'