They were arrested by South Wales Police amid fears of violent clashes at the 1855BST match at Cardiff's Ninian Park ground.

Police launched the operation in St Mary Street

Bluebirds' owner Sam Hammam had hired 25 coaches to bring 1,500 Swans fans to his ground at a cost of �10,000.

Over 5,000 fans turned out for the Welsh derby clash.

But a sting was launched right in the middle of the city - over a mile from the home side's stadium.

Inspector Tony Bishop of South Wales Police said: "As a result of an intelligence-led police operation, South Wales Police stopped a number of men in the St Mary Street and High Street area.

"As a result of searching those men, 20 were arrested for alleged breach of the peace.

Disperse trouble

"Essentially the arrests are a preventative measure and there has been no trouble at all so far."

Police were detaining the group until after the final whistle.

Police had attended to the area, which is home to many bars, and later dispersed.

A police spokesperson later said the Swansea fans arrested had not committed any crime, but were being held under special powers to prevent breach of the peace.

The winner of the cup stands to gain a �100,000 cash prize from the Football Association of Wales.