Thirty football fans brought terror to an afternoon at the races when they fought running battles for 15 minutes.
Groups of Swansea and Cardiff City fans clashed at Newbury Racecourse leading to the closure of bars around the circuit and police descending on the site.
Only three people were arrested - two for affray and one for a public order offence, Thames Valley Police said.
It was not clear why the sets of Welsh fans unleashed their violence at the ground in Berkshire, marring the Newbury Summer Festival.
Race track managing director Stephen Higgins said: "There was sporadic fighting over about 15 minutes with approximately 30 people involved. We have absolutely no idea why it started."
Inspector Phil Rogers of Thames Valley Police said there were on a few minor injuries and no complaints were made to police.
Officers were called at 4.03pm by the race ground staff.
Insp Rogers said: "Two arrests were made for affray and one for a public order offence. The remainder left on buses. It appears to be caused by football supporters from Swansea and Cardiff who met in the ground."
Racegoer Julie Davis told the Racing Post: "It was a disgrace and I won't be coming again. It took so long to get it sorted out. Every time you thought it had stopped it started again. I reckon there were at least 30 people involved. It was scary."
Another punter Denise Mathias told the Post: "It just seemed to be a big fight between football supporters from Cardiff and Swansea. It was terrible. I saw a stocky man covered in blood having his head stamped on."