Police in riot vans were called to Wood Street in Cardiff on Saturday evening following reports of a “large disturbance”.
Trouble erupted between groups of teenagers and young adults, with witnesses reporting that weapons were seen.
Inspector Andy Walbeoff, of South Wales Police , said officers were called to Wood Street at 7.20pm.
It came at the same time as thousands of fans were leaving the Cardiff fanzone.
He said initial reports suggested that those involved had not been in the fanzone, but had been seen in the city centre earlier in the afternoon.
The cause of the disturbance is not yet known.
“The information from witnesses was that they may be in possession of weapons, but no weapons have been recovered,” he said.
Insp Walbeoff added that arrests had been made, but he was not able to confirm numbers.