Trouble broke out before kick-off in the fourth round tie
Twelve people were arrested during crowd trouble at the FA Cup tie between Hull City and Millwall.
There were reports of ripped-out seats used as missiles in Hull's KC Stadium, and police in riot gear tackled the trouble in areas used by Millwall fans.
After the match, which Hull won 2-0, mounted police patrolled the edge of the pitch and thousands of Millwall supporters were delayed from leaving.
A police spokesman said up to 40 seats had been ripped out during the match.
The Humberside Police spokesman added he believed an operation to counter the violence had worked well as officers and stewards prevented the two sets of supporters coming together in the ground.
Trouble first broke out amongst rival supporters in the 18, 639 crowd before kick-off, in a section of the North Stand where Millwall fans were closest to Hull supporters.
There was further trouble during the second-half of the fourth round tie in the opposite end of the North Stand.
Millwall manager Kenny Jackett said: "There were one or two rushes and then towards the end of the game you see the police horses come out.
"I don't think it was off-putting for us."
The arrested fans remain in custody.