Eight people were arrested at Swansea's 1-0 Carling Cup victory over Cardiff after fans clashed with police after the final whistle.
It was the first meeting between the Welsh rivals in almost 10 years and there had been appeals from both sides for restraint from supporters.
Officers from South Wales Police had missiles thrown at them on Tuesday evening during attempts to keep the opposing fans apart at the Liberty Stadium. Five people were arrested in the ground, while a further three were arrested before the game.
Assistant Chief Constable Robert Evans of South Wales Police said: "While the vast majority of spectators were well behaved, a minority made up of several hundred, caused officers and stewards concern throughout the event.
"After the game ended, officers were forced to position themselves in such a way to prevent protagonists clashing. During this, officers were subjected to missile throwing and sustained threats of violence.
"It is of credit to the vast majority of mature fans that they supported police and the clubs and confined their rivalries to the pitch.
"However, the behaviour of others was totally unacceptable. It was thanks to the professionalism of the police officers and the stewards that the violence was contained to a small area and did not spoil the enjoyment of the majority of people."
He added: "The post match investigation will be thorough and done in conjunction with both clubs. Neither the clubs or the police will tolerate this type of behaviour and we will seek to ban those identified from future games.
"On a positive note, when compared to fixtures of previous years, the disorder was limited and confined to a small area, and this is a reflection of the efficiency and effectiveness of the policing operation."
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