Crowd trouble has tarnished Swansea City's 1-0 Carling Cup defeat over Cardiff last night at the Liberty Stadium, with news eight arrests were made.
During the run up to the Cardiff City and Swansea match - the first meeting between the Welsh rival's in almost 10 years - appeals for restraint from supporters were made by both sides.
Despite a heavy police presence, stones and missiles were hurled at officers after the final whistle was blown and Cardiff fans were waiting to leave the stadium on buses.
Five arrests were made on the ground and a further three people were arrested before the game kicked off.
Assistant Chief Constable Robert Evans of South Wales Police said: "Whilst 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."
He added: "It is of credit to the vast majority of mature fans that they supported police and the clubs and confirmed 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."
Police have promised a full investigation will be carried out into the circumstances surrounding the arrests.
Swansea beat their rivals 1-0 after a 57th minute strike from Jordi Gomez, on loan from Spanish side Espanyol.